diff --git a/src/System.Private.CoreLib/shared/System/AppDomain.cs b/src/System.Private.CoreLib/shared/System/AppDomain.cs
index b1a20551b5e..a5399178b15 100644
--- a/src/System.Private.CoreLib/shared/System/AppDomain.cs
+++ b/src/System.Private.CoreLib/shared/System/AppDomain.cs
@@ -399,8 +399,8 @@ public void SetThreadPrincipal(IPrincipal principal)
                     case PrincipalPolicy.UnauthenticatedPrincipal:
                         if (s_getUnauthenticatedPrincipal == null)
                         {
-                            Type type = Type.GetType("System.Security.Principal.GenericPrincipal, System.Security.Claims", throwOnError: true);
-                            MethodInfo mi = type.GetMethod("GetDefaultInstance", BindingFlags.NonPublic | BindingFlags.Static);
+                            Type type = Type.GetType("System.Security.Principal.GenericPrincipal, System.Security.Claims", throwOnError: true)!;
+                            MethodInfo? mi = type.GetMethod("GetDefaultInstance", BindingFlags.NonPublic | BindingFlags.Static);
                             Debug.Assert(mi != null);
                             // Don't throw PNSE if null like for WindowsPrincipal as UnauthenticatedPrincipal should
                             // be available on all platforms.
@@ -414,8 +414,8 @@ public void SetThreadPrincipal(IPrincipal principal)
                     case PrincipalPolicy.WindowsPrincipal:
                         if (s_getWindowsPrincipal == null)
                         {
-                            Type type = Type.GetType("System.Security.Principal.WindowsPrincipal, System.Security.Principal.Windows", throwOnError: true);
-                            MethodInfo mi = type.GetMethod("GetDefaultInstance", BindingFlags.NonPublic | BindingFlags.Static);
+                            Type type = Type.GetType("System.Security.Principal.WindowsPrincipal, System.Security.Principal.Windows", throwOnError: true)!;
+                            MethodInfo? mi = type.GetMethod("GetDefaultInstance", BindingFlags.NonPublic | BindingFlags.Static);
                             if (mi == null)
                             {
                                 throw new PlatformNotSupportedException(SR.PlatformNotSupported_Principal);
diff --git a/src/System.Private.CoreLib/shared/System/Attribute.cs b/src/System.Private.CoreLib/shared/System/Attribute.cs
index 755d7722615..88e741c87b9 100644
--- a/src/System.Private.CoreLib/shared/System/Attribute.cs
+++ b/src/System.Private.CoreLib/shared/System/Attribute.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Diagnostics;
 using System.Reflection;
 
@@ -15,7 +16,7 @@ public abstract partial class Attribute
         protected Attribute() { }
 
 #if !CORERT
-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
         {
             if (obj == null)
                 return false;
@@ -25,7 +26,7 @@ public override bool Equals(object obj)
 
             Type thisType = this.GetType();
             object thisObj = this;
-            object thisResult, thatResult;
+            object? thisResult, thatResult;
 
             while (thisType != typeof(Attribute))
             {
@@ -41,7 +42,7 @@ public override bool Equals(object obj)
                         return false;
                     }
                 }
-                thisType = thisType.BaseType;
+                thisType = thisType.BaseType!;
             }
 
             return true;
@@ -54,11 +55,11 @@ public override int GetHashCode()
             while (type != typeof(Attribute))
             {
                 FieldInfo[] fields = type.GetFields(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.DeclaredOnly);
-                object vThis = null;
+                object? vThis = null;
 
                 for (int i = 0; i < fields.Length; i++)
                 {
-                    object fieldValue = fields[i].GetValue(this);
+                    object? fieldValue = fields[i].GetValue(this);
 
                     // The hashcode of an array ignores the contents of the array, so it can produce 
                     // different hashcodes for arrays with the same contents.
@@ -74,7 +75,7 @@ public override int GetHashCode()
                 if (vThis != null)
                     return vThis.GetHashCode();
 
-                type = type.BaseType;
+                type = type.BaseType!;
             }
 
             return type.GetHashCode();
@@ -82,7 +83,7 @@ public override int GetHashCode()
 #endif
 
         // Compares values of custom-attribute fields.    
-        private static bool AreFieldValuesEqual(object thisValue, object thatValue)
+        private static bool AreFieldValuesEqual(object? thisValue, object? thatValue)
         {
             if (thisValue == null && thatValue == null)
                 return true;
@@ -99,8 +100,8 @@ private static bool AreFieldValuesEqual(object thisValue, object thatValue)
                     return false;
                 }
 
-                Array thisValueArray = thisValue as Array;
-                Array thatValueArray = thatValue as Array;
+                Array thisValueArray = (Array)thisValue;
+                Array thatValueArray = (Array)thatValue;
                 if (thisValueArray.Length != thatValueArray.Length)
                 {
                     return false;
@@ -132,7 +133,7 @@ private static bool AreFieldValuesEqual(object thisValue, object thatValue)
 
         public virtual object TypeId => GetType();
 
-        public virtual bool Match(object obj) => Equals(obj);
+        public virtual bool Match(object? obj) => Equals(obj);
 
         public virtual bool IsDefaultAttribute() => false;
     }
diff --git a/src/System.Private.CoreLib/shared/System/ComponentModel/DefaultValueAttribute.cs b/src/System.Private.CoreLib/shared/System/ComponentModel/DefaultValueAttribute.cs
index 9380064337d..fdf9e90fc4f 100644
--- a/src/System.Private.CoreLib/shared/System/ComponentModel/DefaultValueAttribute.cs
+++ b/src/System.Private.CoreLib/shared/System/ComponentModel/DefaultValueAttribute.cs
@@ -68,7 +68,7 @@ bool TryConvertFromInvariantString(Type? typeToConvert, string? stringValue, out
                     // lazy init reflection objects
                     if (s_convertFromInvariantString == null)
                     {
-                        Type typeDescriptorType = Type.GetType("System.ComponentModel.TypeDescriptor, System.ComponentModel.TypeConverter", throwOnError: false);
+                        Type? typeDescriptorType = Type.GetType("System.ComponentModel.TypeDescriptor, System.ComponentModel.TypeConverter", throwOnError: false);
                         MethodInfo? mi = typeDescriptorType?.GetMethod("ConvertFromInvariantString", BindingFlags.NonPublic | BindingFlags.Static);
                         Volatile.Write(ref s_convertFromInvariantString, mi == null ? new object() : mi.CreateDelegate(typeof(Func<Type, string, object>)));
                     }
diff --git a/src/System.Private.CoreLib/shared/System/DefaultBinder.cs b/src/System.Private.CoreLib/shared/System/DefaultBinder.cs
index dca388dfc87..c1e2bcfeba8 100644
--- a/src/System.Private.CoreLib/shared/System/DefaultBinder.cs
+++ b/src/System.Private.CoreLib/shared/System/DefaultBinder.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Reflection;
 using System.Diagnostics;
 using CultureInfo = System.Globalization.CultureInfo;
@@ -29,13 +30,13 @@ partial class DefaultBinder : Binder
         // The most specific match will be selected.  
         // 
         public sealed override MethodBase BindToMethod(
-            BindingFlags bindingAttr, MethodBase[] match, ref object[] args,
-            ParameterModifier[] modifiers, CultureInfo cultureInfo, string[] names, out object state)
+            BindingFlags bindingAttr, MethodBase[] match, ref object?[] args,
+            ParameterModifier[]? modifiers, CultureInfo? cultureInfo, string[]? names, out object? state)
         {
             if (match == null || match.Length == 0)
                 throw new ArgumentException(SR.Arg_EmptyArray, nameof(match));
 
-            MethodBase[] candidates = (MethodBase[])match.Clone();
+            MethodBase?[] candidates = (MethodBase[])match.Clone();
 
             int i;
             int j;
@@ -52,7 +53,7 @@ public sealed override MethodBase BindToMethod(
 
             for (i = 0; i < candidates.Length; i++)
             {
-                ParameterInfo[] par = candidates[i].GetParametersNoCopy();
+                ParameterInfo[] par = candidates[i]!.GetParametersNoCopy();
 
                 // args.Length + 1 takes into account the possibility of a last paramArray that can be omitted
                 paramOrder[i] = new int[(par.Length > args.Length) ? par.Length : args.Length];
@@ -84,7 +85,7 @@ public sealed override MethodBase BindToMethod(
             {
                 if (args[i] != null)
                 {
-                    argTypes[i] = args[i].GetType();
+                    argTypes[i] = args[i]!.GetType(); //TODO-NULLABLE https://github.com/dotnet/csharplang/issues/2388
                 }
             }
 #endregion
@@ -94,7 +95,7 @@ public sealed override MethodBase BindToMethod(
             int CurIdx = 0;
             bool defaultValueBinding = ((bindingAttr & BindingFlags.OptionalParamBinding) != 0);
 
-            Type paramArrayType = null;
+            Type? paramArrayType;
 
 #region Filter methods by parameter count and type
             for (i = 0; i < candidates.Length; i++)
@@ -106,7 +107,7 @@ public sealed override MethodBase BindToMethod(
                     continue;
 
                 // Validate the parameters.
-                ParameterInfo[] par = candidates[i].GetParametersNoCopy();
+                ParameterInfo[] par = candidates[i]!.GetParametersNoCopy(); // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/34644
 
 #region Match method by parameter count
                 if (par.Length == 0)
@@ -114,7 +115,7 @@ public sealed override MethodBase BindToMethod(
 #region No formal parameters
                     if (args.Length != 0)
                     {
-                        if ((candidates[i].CallingConvention & CallingConventions.VarArgs) == 0)
+                        if ((candidates[i]!.CallingConvention & CallingConventions.VarArgs) == 0) // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/34644
                             continue;
                     }
 
@@ -185,7 +186,7 @@ public sealed override MethodBase BindToMethod(
                 }
 #endregion
 
-                Type pCls = null;
+                Type pCls;
                 int argsToCheck = (paramArrayType != null) ? par.Length - 1 : args.Length;
 
 #region Match method by parameter type
@@ -196,7 +197,7 @@ public sealed override MethodBase BindToMethod(
                     pCls = par[j].ParameterType;
 
                     if (pCls.IsByRef)
-                        pCls = pCls.GetElementType();
+                        pCls = pCls.GetElementType()!;
 
                     // the type is the same
                     if (pCls == argTypes[paramOrder[i][j]])
@@ -275,7 +276,7 @@ public sealed override MethodBase BindToMethod(
                 {
 #region This is a valid routine so we move it up the candidates list
                     paramOrder[CurIdx] = paramOrder[i];
-                    paramArrayTypes[CurIdx] = paramArrayType;
+                    paramArrayTypes[CurIdx] = paramArrayType!;
                     candidates[CurIdx++] = candidates[i];
 #endregion
                 }
@@ -297,7 +298,7 @@ public sealed override MethodBase BindToMethod(
 
                 // If the parameters and the args are not the same length or there is a paramArray
                 //  then we need to create a argument array.
-                ParameterInfo[] parms = candidates[0].GetParametersNoCopy();
+                ParameterInfo[] parms = candidates[0]!.GetParametersNoCopy();
 
                 if (parms.Length == args.Length)
                 {
@@ -313,7 +314,7 @@ public sealed override MethodBase BindToMethod(
                 }
                 else if (parms.Length > args.Length)
                 {
-                    object[] objs = new object[parms.Length];
+                    object?[] objs = new object[parms.Length];
 
                     for (i = 0; i < args.Length; i++)
                         objs[i] = args[i];
@@ -331,7 +332,7 @@ public sealed override MethodBase BindToMethod(
                 }
                 else
                 {
-                    if ((candidates[0].CallingConvention & CallingConventions.VarArgs) == 0)
+                    if ((candidates[0]!.CallingConvention & CallingConventions.VarArgs) == 0)
                     {
                         object[] objs = new object[parms.Length];
                         int paramArrayPos = parms.Length - 1;
@@ -343,7 +344,7 @@ public sealed override MethodBase BindToMethod(
                 }
 #endregion
 
-                return candidates[0];
+                return candidates[0]!;
             }
 
             int currentMin = 0;
@@ -351,8 +352,8 @@ public sealed override MethodBase BindToMethod(
             for (i = 1; i < CurIdx; i++)
             {
 #region Walk all of the methods looking the most specific method to invoke
-                int newMin = FindMostSpecificMethod(candidates[currentMin], paramOrder[currentMin], paramArrayTypes[currentMin],
-                                                    candidates[i], paramOrder[i], paramArrayTypes[i], argTypes, args);
+                int newMin = FindMostSpecificMethod(candidates[currentMin]!, paramOrder[currentMin], paramArrayTypes[currentMin],
+                                                    candidates[i]!, paramOrder[i], paramArrayTypes[i], argTypes, args);
 
                 if (newMin == 0)
                 {
@@ -378,7 +379,7 @@ public sealed override MethodBase BindToMethod(
 
             // If the parameters and the args are not the same length or there is a paramArray
             //  then we need to create a argument array.
-            ParameterInfo[] parameters = candidates[currentMin].GetParametersNoCopy();
+            ParameterInfo[] parameters = candidates[currentMin]!.GetParametersNoCopy();
             if (parameters.Length == args.Length)
             {
                 if (paramArrayTypes[currentMin] != null)
@@ -393,7 +394,7 @@ public sealed override MethodBase BindToMethod(
             }
             else if (parameters.Length > args.Length)
             {
-                object[] objs = new object[parameters.Length];
+                object?[] objs = new object[parameters.Length];
 
                 for (i = 0; i < args.Length; i++)
                     objs[i] = args[i];
@@ -414,7 +415,7 @@ public sealed override MethodBase BindToMethod(
             }
             else
             {
-                if ((candidates[currentMin].CallingConvention & CallingConventions.VarArgs) == 0)
+                if ((candidates[currentMin]!.CallingConvention & CallingConventions.VarArgs) == 0)
                 {
                     object[] objs = new object[parameters.Length];
                     int paramArrayPos = parameters.Length - 1;
@@ -425,13 +426,13 @@ public sealed override MethodBase BindToMethod(
                 }
             }
 
-            return candidates[currentMin];
+            return candidates[currentMin]!;
         }
 
 
         // Given a set of fields that match the base criteria, select a field.
         // if value is null then we have no way to select a field
-        public sealed override FieldInfo BindToField(BindingFlags bindingAttr, FieldInfo[] match, object value, CultureInfo cultureInfo)
+        public sealed override FieldInfo BindToField(BindingFlags bindingAttr, FieldInfo[] match, object value, CultureInfo? cultureInfo)
         {
             if (match == null)
             {
@@ -442,7 +443,7 @@ public sealed override FieldInfo BindToField(BindingFlags bindingAttr, FieldInfo
             // Find the method that match...
             int CurIdx = 0;
 
-            Type valueType = null;
+            Type valueType;
 
             FieldInfo[] candidates = (FieldInfo[])match.Clone();
 
@@ -521,7 +522,7 @@ public sealed override FieldInfo BindToField(BindingFlags bindingAttr, FieldInfo
         // Given a set of methods that match the base criteria, select a method based
         // upon an array of types.  This method should return null if no method matchs
         // the criteria.
-        public sealed override MethodBase SelectMethod(BindingFlags bindingAttr, MethodBase[] match, Type[] types, ParameterModifier[] modifiers)
+        public sealed override MethodBase? SelectMethod(BindingFlags bindingAttr, MethodBase[] match, Type[] types, ParameterModifier[]? modifiers)
         {
             int i;
             int j;
@@ -557,7 +558,7 @@ public sealed override MethodBase SelectMethod(BindingFlags bindingAttr, MethodB
                     if (pCls == typeof(object))
                         continue;
 
-                    Type type = types[j];
+                    Type? type = types[j];
                     if (type is SignatureType signatureType)
                     {
                         if (!(candidates[i] is MethodInfo methodInfo))
@@ -614,8 +615,8 @@ public sealed override MethodBase SelectMethod(BindingFlags bindingAttr, MethodB
         }
 
         // Given a set of properties that match the base criteria, select one.
-        public sealed override PropertyInfo SelectProperty(BindingFlags bindingAttr, PropertyInfo[] match, Type returnType,
-                    Type[] indexes, ParameterModifier[] modifiers)
+        public sealed override PropertyInfo? SelectProperty(BindingFlags bindingAttr, PropertyInfo[] match, Type? returnType,
+                    Type[]? indexes, ParameterModifier[]? modifiers)
         {
             // Allow a null indexes array. But if it is not null, every element must be non-null as well.
             if (indexes != null)
@@ -732,7 +733,7 @@ public sealed override PropertyInfo SelectProperty(BindingFlags bindingAttr, Pro
         // ChangeType
         // The default binder doesn't support any change type functionality.
         // This is because the default is built into the low level invoke code.
-        public override object ChangeType(object value, Type type, CultureInfo cultureInfo)
+        public override object ChangeType(object value, Type type, CultureInfo? cultureInfo)
         {
             throw new NotSupportedException(SR.NotSupported_ChangeType);
         }
@@ -771,7 +772,7 @@ public sealed override void ReorderArgumentArray(ref object[] args, object state
 
         // Return any exact bindings that may exist. (This method is not defined on the
         //  Binder and is used by RuntimeType.)
-        public static MethodBase ExactBinding(MethodBase[] match, Type[] types, ParameterModifier[] modifiers)
+        public static MethodBase? ExactBinding(MethodBase[] match, Type[] types, ParameterModifier[]? modifiers)
         {
             if (match == null)
                 throw new ArgumentNullException(nameof(match));
@@ -814,12 +815,12 @@ public static MethodBase ExactBinding(MethodBase[] match, Type[] types, Paramete
 
         // Return any exact bindings that may exist. (This method is not defined on the
         //  Binder and is used by RuntimeType.)
-        public static PropertyInfo ExactPropertyBinding(PropertyInfo[] match, Type returnType, Type[] types, ParameterModifier[] modifiers)
+        public static PropertyInfo? ExactPropertyBinding(PropertyInfo[] match, Type? returnType, Type[]? types, ParameterModifier[]? modifiers)
         {
             if (match == null)
                 throw new ArgumentNullException(nameof(match));
 
-            PropertyInfo bestMatch = null;
+            PropertyInfo? bestMatch = null;
             int typesLength = (types != null) ? types.Length : 0;
             for (int i = 0; i < match.Length; i++)
             {
@@ -830,7 +831,7 @@ public static PropertyInfo ExactPropertyBinding(PropertyInfo[] match, Type retur
                     Type pCls = par[j].ParameterType;
 
                     // If the classes  exactly match continue
-                    if (pCls != types[j])
+                    if (pCls != types![j])
                         break;
                 }
                 if (j < typesLength)
@@ -846,9 +847,9 @@ public static PropertyInfo ExactPropertyBinding(PropertyInfo[] match, Type retur
             return bestMatch;
         }
 
-        private static int FindMostSpecific(ParameterInfo[] p1, int[] paramOrder1, Type paramArrayType1,
-                                            ParameterInfo[] p2, int[] paramOrder2, Type paramArrayType2,
-                                            Type[] types, object[] args)
+        private static int FindMostSpecific(ParameterInfo[] p1, int[] paramOrder1, Type? paramArrayType1,
+                                            ParameterInfo[] p2, int[] paramOrder2, Type? paramArrayType2,
+                                            Type[] types, object?[]? args)
         {
             // A method using params is always less specific than one not using params
             if (paramArrayType1 != null && paramArrayType2 == null) return 2;
@@ -923,7 +924,7 @@ private static int FindMostSpecific(ParameterInfo[] p1, int[] paramOrder1, Type
             }
         }
 
-        private static int FindMostSpecificType(Type c1, Type c2, Type t)
+        private static int FindMostSpecificType(Type c1, Type c2, Type? t)
         {
             // If the two types are exact move on...
             if (c1 == c2)
@@ -953,22 +954,22 @@ private static int FindMostSpecificType(Type c1, Type c2, Type t)
             {
                 if (c1.IsByRef && c2.IsByRef)
                 {
-                    c1 = c1.GetElementType();
-                    c2 = c2.GetElementType();
+                    c1 = c1.GetElementType()!;
+                    c2 = c2.GetElementType()!;
                 }
                 else if (c1.IsByRef)
                 {
                     if (c1.GetElementType() == c2)
                         return 2;
 
-                    c1 = c1.GetElementType();
+                    c1 = c1.GetElementType()!;
                 }
-                else
+                else // if (c2.IsByRef)
                 {
                     if (c2.GetElementType() == c1)
                         return 1;
 
-                    c2 = c2.GetElementType();
+                    c2 = c2.GetElementType()!;
                 }
             }
 
@@ -997,9 +998,9 @@ private static int FindMostSpecificType(Type c1, Type c2, Type t)
             }
         }
 
-        private static int FindMostSpecificMethod(MethodBase m1, int[] paramOrder1, Type paramArrayType1,
-                                                  MethodBase m2, int[] paramOrder2, Type paramArrayType2,
-                                                  Type[] types, object[] args)
+        private static int FindMostSpecificMethod(MethodBase m1, int[] paramOrder1, Type? paramArrayType1,
+                                                  MethodBase m2, int[] paramOrder2, Type? paramArrayType2,
+                                                  Type[] types, object?[]? args)
         {
             // Find the most specific method based on the parameters.
             int res = FindMostSpecific(m1.GetParametersNoCopy(), paramOrder1, paramArrayType1,
@@ -1013,8 +1014,8 @@ private static int FindMostSpecificMethod(MethodBase m1, int[] paramOrder1, Type
             if (CompareMethodSig(m1, m2))
             {
                 // Determine the depth of the declaring types for both methods.
-                int hierarchyDepth1 = GetHierarchyDepth(m1.DeclaringType);
-                int hierarchyDepth2 = GetHierarchyDepth(m2.DeclaringType);
+                int hierarchyDepth1 = GetHierarchyDepth(m1.DeclaringType!);
+                int hierarchyDepth2 = GetHierarchyDepth(m2.DeclaringType!);
 
                 // The most derived method is the most specific one.
                 if (hierarchyDepth1 == hierarchyDepth2)
@@ -1040,8 +1041,8 @@ private static int FindMostSpecificField(FieldInfo cur1, FieldInfo cur2)
             // Check to see if the fields have the same name.
             if (cur1.Name == cur2.Name)
             {
-                int hierarchyDepth1 = GetHierarchyDepth(cur1.DeclaringType);
-                int hierarchyDepth2 = GetHierarchyDepth(cur2.DeclaringType);
+                int hierarchyDepth1 = GetHierarchyDepth(cur1.DeclaringType!);
+                int hierarchyDepth2 = GetHierarchyDepth(cur2.DeclaringType!);
 
                 if (hierarchyDepth1 == hierarchyDepth2)
                 {
@@ -1063,8 +1064,8 @@ private static int FindMostSpecificProperty(PropertyInfo cur1, PropertyInfo cur2
             // Check to see if the fields have the same name.
             if (cur1.Name == cur2.Name)
             {
-                int hierarchyDepth1 = GetHierarchyDepth(cur1.DeclaringType);
-                int hierarchyDepth2 = GetHierarchyDepth(cur2.DeclaringType);
+                int hierarchyDepth1 = GetHierarchyDepth(cur1.DeclaringType!);
+                int hierarchyDepth2 = GetHierarchyDepth(cur2.DeclaringType!);
 
                 if (hierarchyDepth1 == hierarchyDepth2)
                 {
@@ -1102,7 +1103,7 @@ private static int GetHierarchyDepth(Type t)
         {
             int depth = 0;
 
-            Type currentType = t;
+            Type? currentType = t;
             do
             {
                 depth++;
@@ -1112,16 +1113,16 @@ private static int GetHierarchyDepth(Type t)
             return depth;
         }
 
-        internal static MethodBase FindMostDerivedNewSlotMeth(MethodBase[] match, int cMatches)
+        internal static MethodBase? FindMostDerivedNewSlotMeth(MethodBase[] match, int cMatches)
         {
             int deepestHierarchy = 0;
-            MethodBase methWithDeepestHierarchy = null;
+            MethodBase? methWithDeepestHierarchy = null;
 
             for (int i = 0; i < cMatches; i++)
             {
                 // Calculate the depth of the hierarchy of the declaring type of the
                 // current method.
-                int currentHierarchyDepth = GetHierarchyDepth(match[i].DeclaringType);
+                int currentHierarchyDepth = GetHierarchyDepth(match[i].DeclaringType!);
 
                 // The two methods have the same name, signature, and hierarchy depth.
                 // This can only happen if at least one is vararg or generic.
@@ -1143,9 +1144,9 @@ internal static MethodBase FindMostDerivedNewSlotMeth(MethodBase[] match, int cM
 
         // This method will sort the vars array into the mapping order stored
         //  in the paramOrder array.
-        private static void ReorderParams(int[] paramOrder, object[] vars)
+        private static void ReorderParams(int[] paramOrder, object?[] vars)
         {
-            object[] varsCopy = new object[vars.Length];
+            object?[] varsCopy = new object[vars.Length];
             for (int i = 0; i < vars.Length; i++)
                 varsCopy[i] = vars[i];
 
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/StackTrace.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/StackTrace.cs
index fccd9e1cf06..7b32f063f57 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/StackTrace.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/StackTrace.cs
@@ -236,7 +236,7 @@ internal string ToString(TraceFormat traceFormat)
                     if (declaringType != null)
                     {
                         // Append t.FullName, replacing '+' with '.'
-                        string fullName = declaringType.FullName;
+                        string fullName = declaringType.FullName!;
                         for (int i = 0; i < fullName.Length; i++)
                         {
                             char ch = fullName[i];
@@ -386,7 +386,7 @@ private static bool TryResolveStateMachineMethod(ref MethodBase method, out Type
                     // of the original method. Non-iterator async state machines resolve directly to their builder methods
                     // so aren't marked as changed.
                     method = candidateMethod;
-                    declaringType = candidateMethod.DeclaringType;
+                    declaringType = candidateMethod.DeclaringType!;
                     return foundIteratorAttribute;
                 }
             }
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs
index 5a662570440..c0416fcb2cb 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs
@@ -3078,7 +3078,7 @@ internal static Attribute GetCustomAttributeHelper(Type type, Type attributeType
 
 #if (!ES_BUILD_PCL && !ES_BUILD_PN)
             // In the reflection only context, we have to do things by hand.
-            string fullTypeNameToFind = attributeType.FullName;
+            string fullTypeNameToFind = attributeType.FullName!;
 
 #if EVENT_SOURCE_LEGACY_NAMESPACE_SUPPORT
             fullTypeNameToFind = fullTypeNameToFind.Replace("System.Diagnostics.Eventing", "System.Diagnostics.Tracing");
@@ -3086,7 +3086,7 @@ internal static Attribute GetCustomAttributeHelper(Type type, Type attributeType
 
             foreach (CustomAttributeData data in CustomAttributeData.GetCustomAttributes(member))
             {
-                if (AttributeTypeNamesMatch(attributeType, data.Constructor.ReflectedType))
+                if (AttributeTypeNamesMatch(attributeType, data.Constructor.ReflectedType!))
                 {
                     Attribute? attr = null;
 
@@ -3094,7 +3094,7 @@ internal static Attribute GetCustomAttributeHelper(Type type, Type attributeType
 
                     if (data.ConstructorArguments.Count == 1)
                     {
-                        attr = (Attribute?)Activator.CreateInstance(attributeType, new object[] { data.ConstructorArguments[0].Value });
+                        attr = (Attribute?)Activator.CreateInstance(attributeType, new object?[] { data.ConstructorArguments[0].Value });
                     }
                     else if (data.ConstructorArguments.Count == 0)
                     {
@@ -3107,8 +3107,8 @@ internal static Attribute GetCustomAttributeHelper(Type type, Type attributeType
 
                         foreach (CustomAttributeNamedArgument namedArgument in data.NamedArguments)
                         {
-                            PropertyInfo p = t.GetProperty(namedArgument.MemberInfo.Name, BindingFlags.Public | BindingFlags.Instance);
-                            object value = namedArgument.TypedValue.Value;
+                            PropertyInfo p = t.GetProperty(namedArgument.MemberInfo.Name, BindingFlags.Public | BindingFlags.Instance)!; // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/26761
+                            object value = namedArgument.TypedValue.Value!;
 
                             if (p.PropertyType.IsEnum)
                             {
@@ -3150,8 +3150,8 @@ private static bool AttributeTypeNamesMatch(Type attributeType, Type reflectedAt
                     // are the typenames equal and the namespaces under "Diagnostics.Tracing" (typically
                     // either Microsoft.Diagnostics.Tracing or System.Diagnostics.Tracing)?
                     string.Equals(attributeType.Name, reflectedAttributeType.Name, StringComparison.Ordinal) &&
-                    attributeType.Namespace.EndsWith("Diagnostics.Tracing", StringComparison.Ordinal) &&
-                    (reflectedAttributeType.Namespace.EndsWith("Diagnostics.Tracing", StringComparison.Ordinal)
+                    attributeType.Namespace!.EndsWith("Diagnostics.Tracing", StringComparison.Ordinal) &&
+                    (reflectedAttributeType.Namespace!.EndsWith("Diagnostics.Tracing", StringComparison.Ordinal)
 #if EVENT_SOURCE_LEGACY_NAMESPACE_SUPPORT
                      || reflectedAttributeType.Namespace.EndsWith("Diagnostics.Eventing", StringComparison.Ordinal)
 #endif
@@ -3261,7 +3261,7 @@ private static bool AttributeTypeNamesMatch(Type attributeType, Type reflectedAt
                 foreach (var providerEnumKind in new string[] { "Keywords", "Tasks", "Opcodes" })
 #endif
                 {
-                    Type nestedType = eventSourceType.GetNestedType(providerEnumKind);
+                    Type? nestedType = eventSourceType.GetNestedType(providerEnumKind);
                     if (nestedType != null)
                     {
                         if (eventSourceType.IsAbstract())
@@ -3412,7 +3412,7 @@ private static bool AttributeTypeNamesMatch(Type attributeType, Type reflectedAt
                             manifest.StartEvent(eventName, eventAttribute);
                             for (int fieldIdx = 0; fieldIdx < args.Length; fieldIdx++)
                             {
-                                manifest.AddEventParameter(args[fieldIdx].ParameterType, args[fieldIdx].Name);
+                                manifest.AddEventParameter(args[fieldIdx].ParameterType, args[fieldIdx].Name!);
                             }
                             manifest.EndEvent();
                         }
@@ -3739,7 +3739,7 @@ private static int GetHelperCallFirstArg(MethodInfo method)
 #if ES_BUILD_STANDALONE
             (new ReflectionPermission(ReflectionPermissionFlag.MemberAccess)).Assert();
 #endif
-            byte[] instrs = method.GetMethodBody().GetILAsByteArray();
+            byte[] instrs = method.GetMethodBody().GetILAsByteArray()!;
             int retVal = -1;
             for (int idx = 0; idx < instrs.Length;)
             {
@@ -4799,7 +4799,7 @@ public ReadOnlyCollection<string>? PayloadNames
                     Debug.Assert(m_eventSource.m_eventData != null);
                     foreach (var parameter in m_eventSource.m_eventData[EventId].Parameters)
                     {
-                        names.Add(parameter.Name);
+                        names.Add(parameter.Name!);
                     }
 
                     m_payloadNames = new ReadOnlyCollection<string>(names);
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/StubEnvironment.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/StubEnvironment.cs
index 7487c0f0431..63e975c1d0d 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/StubEnvironment.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/StubEnvironment.cs
@@ -221,7 +221,7 @@ static class ReflectionExtensions
         public static bool IsSealed(this Type type) { return type.IsSealed; }
         public static bool IsValueType(this Type type) { return type.IsValueType; }
         public static bool IsGenericType(this Type type) { return type.IsGenericType; }
-        public static Type BaseType(this Type type) { return type.BaseType; }
+        public static Type? BaseType(this Type type) { return type.BaseType; }
         public static Assembly Assembly(this Type type) { return type.Assembly; }
         public static TypeCode GetTypeCode(this Type type) { return Type.GetTypeCode(type); }
 
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/PropertyValue.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/PropertyValue.cs
index 35fe7609f18..0b3c67f7955 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/PropertyValue.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/PropertyValue.cs
@@ -176,7 +176,7 @@ public int ScalarLength
         /// </summary>
         public static Func<PropertyValue, PropertyValue> GetPropertyGetter(PropertyInfo property)
         {
-            if (property.DeclaringType.GetTypeInfo().IsValueType)
+            if (property.DeclaringType!.GetTypeInfo().IsValueType)
                 return GetBoxedValueTypePropertyGetter(property);
             else
                 return GetReferenceTypePropertyGetter(property);
@@ -209,7 +209,7 @@ private static Func<PropertyValue, PropertyValue> GetBoxedValueTypePropertyGette
         /// <returns></returns>
         private static Func<PropertyValue, PropertyValue> GetReferenceTypePropertyGetter(PropertyInfo property)
         {
-            var helper = (TypeHelper)Activator.CreateInstance(typeof(ReferenceTypeHelper<>).MakeGenericType(property.DeclaringType))!;
+            var helper = (TypeHelper)Activator.CreateInstance(typeof(ReferenceTypeHelper<>).MakeGenericType(property.DeclaringType!))!;
             return helper.GetPropertyGetter(property);
         }
 
@@ -224,7 +224,7 @@ abstract class TypeHelper
 
             protected Delegate GetGetMethod(PropertyInfo property, Type propertyType)
             {
-                return property.GetMethod.CreateDelegate(typeof(Func<,>).MakeGenericType(property.DeclaringType, propertyType));
+                return property.GetMethod!.CreateDelegate(typeof(Func<,>).MakeGenericType(property.DeclaringType!, propertyType));
             }
         }
 
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/SimpleTypeInfos.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/SimpleTypeInfos.cs
index dc8be3e4d36..aaa0a30d938 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/SimpleTypeInfos.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/SimpleTypeInfos.cs
@@ -278,7 +278,7 @@ public NullableTypeInfo(Type type, List<Type> recursionCheck)
             var typeArgs = type.GenericTypeArguments;
             Debug.Assert(typeArgs.Length == 1);
             this.valueInfo = TraceLoggingTypeInfo.GetInstance(typeArgs[0], recursionCheck);
-            this.valueGetter = PropertyValue.GetPropertyGetter(type.GetTypeInfo().GetDeclaredProperty("Value"));
+            this.valueGetter = PropertyValue.GetPropertyGetter(type.GetTypeInfo().GetDeclaredProperty("Value")!);
         }
 
         public override void WriteMetadata(
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/Statics.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/Statics.cs
index 56d53c1d189..8fa50ac1282 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/Statics.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/Statics.cs
@@ -387,15 +387,15 @@ public static IEnumerable<PropertyInfo> GetProperties(Type type)
             return result;
         }
 
-        public static MethodInfo GetGetMethod(PropertyInfo propInfo)
+        public static MethodInfo? GetGetMethod(PropertyInfo propInfo)
         {
-            MethodInfo result = propInfo.GetGetMethod();
+            MethodInfo? result = propInfo.GetGetMethod();
             return result;
         }
 
-        public static MethodInfo GetDeclaredStaticMethod(Type declaringType, string name)
+        public static MethodInfo? GetDeclaredStaticMethod(Type declaringType, string name)
         {
-            MethodInfo result;
+            MethodInfo? result;
 #if (ES_BUILD_PCL || ES_BUILD_PN)
             result = declaringType.GetTypeInfo().GetDeclaredMethod(name);
 #else
@@ -506,9 +506,9 @@ public static Type[] GetGenericArguments(Type type)
             return elementType;
         }
 
-        public static bool IsGenericMatch(Type type, object openType)
+        public static bool IsGenericMatch(Type type, object? openType)
         {
-            return type.IsGenericType() && type.GetGenericTypeDefinition() == (Type)openType;
+            return type.IsGenericType() && type.GetGenericTypeDefinition() == (Type?)openType;
         }
 
         public static Delegate CreateDelegate(Type delegateType, MethodInfo methodInfo)
@@ -548,7 +548,7 @@ public static TraceLoggingTypeInfo CreateDefaultTypeInfo(
             }
             else if (dataType.IsArray)
             {
-                var elementType = dataType.GetElementType();
+                Type elementType = dataType.GetElementType()!;
                 if (elementType == typeof(bool))
                 {
                     result = ScalarArrayTypeInfo.Boolean();
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/TraceLoggingEventTypes.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/TraceLoggingEventTypes.cs
index d894acbb755..8963f5b91c5 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/TraceLoggingEventTypes.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/TraceLoggingEventTypes.cs
@@ -115,7 +115,7 @@ internal TraceLoggingEventTypes(
                 this.opcode = Statics.Combine((int)typeInfo.Opcode, this.opcode);
                 this.keywords |= typeInfo.Keywords;
                 var paramName = paramInfos[i].Name;
-                if (Statics.ShouldOverrideFieldName(paramName))
+                if (Statics.ShouldOverrideFieldName(paramName!))
                 {
                     paramName = typeInfo.Name;
                 }
@@ -261,7 +261,7 @@ private static string[] MakeParamNameArray(
             string[] paramNames = new string[paramInfos.Length];
             for (int i = 0; i < paramNames.Length; i++)
             {
-                paramNames[i] = paramInfos[i].Name;
+                paramNames[i] = paramInfos[i].Name!;
             }
 
             return paramNames;
diff --git a/src/System.Private.CoreLib/shared/System/Environment.Unix.cs b/src/System.Private.CoreLib/shared/System/Environment.Unix.cs
index d74086b0fc4..8f5fede529f 100644
--- a/src/System.Private.CoreLib/shared/System/Environment.Unix.cs
+++ b/src/System.Private.CoreLib/shared/System/Environment.Unix.cs
@@ -79,8 +79,8 @@ private static string GetFolderPathCore(SpecialFolder folder, SpecialFolderOptio
                 // TODO #11151: Replace with Directory.CreateDirectory once we have access to System.IO.FileSystem here.
                 Func<string, object> createDirectory = LazyInitializer.EnsureInitialized(ref s_directoryCreateDirectory, () =>
                 {
-                    Type dirType = Type.GetType("System.IO.Directory, System.IO.FileSystem, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", throwOnError: true);
-                    MethodInfo mi = dirType.GetTypeInfo().GetDeclaredMethod("CreateDirectory");
+                    Type dirType = Type.GetType("System.IO.Directory, System.IO.FileSystem, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", throwOnError: true)!;
+                    MethodInfo mi = dirType.GetTypeInfo().GetDeclaredMethod("CreateDirectory")!;
                     return (Func<string, object>)mi.CreateDelegate(typeof(Func<string, object>));
                 })!; // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/26761
                 createDirectory(path);
diff --git a/src/System.Private.CoreLib/shared/System/Environment.Win32.cs b/src/System.Private.CoreLib/shared/System/Environment.Win32.cs
index bc32146cf9d..2f277736320 100644
--- a/src/System.Private.CoreLib/shared/System/Environment.Win32.cs
+++ b/src/System.Private.CoreLib/shared/System/Environment.Win32.cs
@@ -411,7 +411,7 @@ public static string GetFolderPath(SpecialFolder folder, SpecialFolderOption opt
             {
                 if (s_winRTFolderPathsGetFolderPath == null)
                 {
-                    Type winRtFolderPathsType = Type.GetType("System.WinRTFolderPaths, System.Runtime.WindowsRuntime, Version=4.0.14.0, Culture=neutral, PublicKeyToken=b77a5c561934e089", throwOnError: false);
+                    Type? winRtFolderPathsType = Type.GetType("System.WinRTFolderPaths, System.Runtime.WindowsRuntime, Version=4.0.14.0, Culture=neutral, PublicKeyToken=b77a5c561934e089", throwOnError: false);
                     MethodInfo? getFolderPathsMethod = winRtFolderPathsType?.GetMethod("GetFolderPath", BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Static, null, new Type[] { typeof(SpecialFolder), typeof(SpecialFolderOption) }, null);
                     var d = (Func<SpecialFolder, SpecialFolderOption, string>?)getFolderPathsMethod?.CreateDelegate(typeof(Func<SpecialFolder, SpecialFolderOption, string>));
                     s_winRTFolderPathsGetFolderPath = d ?? delegate { return string.Empty; };
diff --git a/src/System.Private.CoreLib/shared/System/Environment.cs b/src/System.Private.CoreLib/shared/System/Environment.cs
index 3691c0de2b7..6e9a6bcf6dc 100644
--- a/src/System.Private.CoreLib/shared/System/Environment.cs
+++ b/src/System.Private.CoreLib/shared/System/Environment.cs
@@ -162,7 +162,7 @@ public static long WorkingSet
                 // we do this to avoid duplicating the Windows, Linux, macOS, and potentially other platform-specific implementations
                 // present in Process.  If it proves important, we could look at separating that functionality out of Process into
                 // Common files which could also be included here.
-                Type processType = Type.GetType("System.Diagnostics.Process, System.Diagnostics.Process, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", throwOnError: false);
+                Type? processType = Type.GetType("System.Diagnostics.Process, System.Diagnostics.Process, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", throwOnError: false);
                 IDisposable? currentProcess = processType?.GetMethod("GetCurrentProcess")?.Invoke(null, BindingFlags.DoNotWrapExceptions, null, null, null) as IDisposable;
                 if (currentProcess != null)
                 {
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/Assembly.cs b/src/System.Private.CoreLib/shared/System/Reflection/Assembly.cs
index 4722ed3fc9e..bb3dc2117a0 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/Assembly.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/Assembly.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.IO;
 using System.Globalization;
 using System.Collections.Generic;
@@ -73,19 +74,19 @@ public virtual Type[] GetTypes()
         public virtual Type[] GetExportedTypes() { throw NotImplemented.ByDesign; }
         public virtual Type[] GetForwardedTypes() { throw NotImplemented.ByDesign; }
 
-        public virtual string CodeBase { get { throw NotImplemented.ByDesign; } }
-        public virtual MethodInfo EntryPoint { get { throw NotImplemented.ByDesign; } }
-        public virtual string FullName { get { throw NotImplemented.ByDesign; } }
+        public virtual string? CodeBase { get { throw NotImplemented.ByDesign; } }
+        public virtual MethodInfo? EntryPoint { get { throw NotImplemented.ByDesign; } }
+        public virtual string? FullName { get { throw NotImplemented.ByDesign; } }
         public virtual string ImageRuntimeVersion { get { throw NotImplemented.ByDesign; } }
         public virtual bool IsDynamic => false;
         public virtual string Location { get { throw NotImplemented.ByDesign; } }
         public virtual bool ReflectionOnly { get { throw NotImplemented.ByDesign; } }
         public virtual bool IsCollectible => true;
 
-        public virtual ManifestResourceInfo GetManifestResourceInfo(string resourceName) { throw NotImplemented.ByDesign; }
+        public virtual ManifestResourceInfo? GetManifestResourceInfo(string resourceName) { throw NotImplemented.ByDesign; }
         public virtual string[] GetManifestResourceNames() { throw NotImplemented.ByDesign; }
-        public virtual Stream GetManifestResourceStream(string name) { throw NotImplemented.ByDesign; }
-        public virtual Stream GetManifestResourceStream(Type type, string name) { throw NotImplemented.ByDesign; }
+        public virtual Stream? GetManifestResourceStream(string name) { throw NotImplemented.ByDesign; }
+        public virtual Stream? GetManifestResourceStream(Type type, string name) { throw NotImplemented.ByDesign; }
 
         public bool IsFullyTrusted => true;
 
@@ -106,9 +107,9 @@ public virtual Type[] GetTypes()
 
         public virtual string EscapedCodeBase => AssemblyName.EscapeCodeBase(CodeBase);
 
-        public object CreateInstance(string typeName) => CreateInstance(typeName, false, BindingFlags.Public | BindingFlags.Instance, binder: null, args: null, culture: null, activationAttributes: null);
-        public object CreateInstance(string typeName, bool ignoreCase) => CreateInstance(typeName, ignoreCase, BindingFlags.Public | BindingFlags.Instance, binder: null, args: null, culture: null, activationAttributes: null);
-        public virtual object CreateInstance(string typeName, bool ignoreCase, BindingFlags bindingAttr, Binder binder, object[] args, CultureInfo culture, object[] activationAttributes)
+        public object? CreateInstance(string typeName) => CreateInstance(typeName, false, BindingFlags.Public | BindingFlags.Instance, binder: null, args: null, culture: null, activationAttributes: null);
+        public object? CreateInstance(string typeName, bool ignoreCase) => CreateInstance(typeName, ignoreCase, BindingFlags.Public | BindingFlags.Instance, binder: null, args: null, culture: null, activationAttributes: null);
+        public virtual object? CreateInstance(string typeName, bool ignoreCase, BindingFlags bindingAttr, Binder? binder, object[]? args, CultureInfo? culture, object[]? activationAttributes)
         {
             Type t = GetType(typeName, throwOnError: false, ignoreCase: ignoreCase);
             if (t == null)
@@ -119,7 +120,7 @@ public virtual object CreateInstance(string typeName, bool ignoreCase, BindingFl
 
         public virtual event ModuleResolveEventHandler ModuleResolve { add { throw NotImplemented.ByDesign; } remove { throw NotImplemented.ByDesign; } }
 
-        public virtual Module ManifestModule { get { throw NotImplemented.ByDesign; } }
+        public virtual Module? ManifestModule { get { throw NotImplemented.ByDesign; } }
         public virtual Module GetModule(string name) { throw NotImplemented.ByDesign; }
 
         public Module[] GetModules() => GetModules(getResourceModules: false);
@@ -132,17 +133,17 @@ public virtual object CreateInstance(string typeName, bool ignoreCase, BindingFl
         public virtual AssemblyName[] GetReferencedAssemblies() { throw NotImplemented.ByDesign; }
 
         public virtual Assembly GetSatelliteAssembly(CultureInfo culture) { throw NotImplemented.ByDesign; }
-        public virtual Assembly GetSatelliteAssembly(CultureInfo culture, Version version) { throw NotImplemented.ByDesign; }
+        public virtual Assembly GetSatelliteAssembly(CultureInfo culture, Version? version) { throw NotImplemented.ByDesign; }
 
-        public virtual FileStream GetFile(string name) { throw NotImplemented.ByDesign; }
+        public virtual FileStream? GetFile(string name) { throw NotImplemented.ByDesign; }
         public virtual FileStream[] GetFiles() => GetFiles(getResourceModules: false);
         public virtual FileStream[] GetFiles(bool getResourceModules) { throw NotImplemented.ByDesign; }
 
         public virtual void GetObjectData(SerializationInfo info, StreamingContext context) { throw NotImplemented.ByDesign; }
 
-        public override string ToString()
+        public override string? ToString()
         {
-            string displayName = FullName;
+            string? displayName = FullName;
             if (displayName == null)
                 return base.ToString();
             else
@@ -155,11 +156,11 @@ public override string ToString()
         public virtual bool GlobalAssemblyCache { get { throw NotImplemented.ByDesign; } }
         public virtual long HostContext { get { throw NotImplemented.ByDesign; } }
 
-        public override bool Equals(object o) => base.Equals(o);
+        public override bool Equals(object? o) => base.Equals(o);
         public override int GetHashCode() => base.GetHashCode();
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static bool operator ==(Assembly left, Assembly right)
+        public static bool operator ==(Assembly? left, Assembly? right)
         {
             // Test "right" first to allow branch elimination when inlined for null checks (== null)
             // so it can become a simple test
@@ -170,7 +171,7 @@ public override string ToString()
             }
 
             // Try fast reference equality and opposite null check prior to calling the slower virtual Equals
-            if ((object)left == (object)right)
+            if ((object?)left == (object)right)
             {
                 return true;
             }
@@ -178,14 +179,14 @@ public override string ToString()
             return (left is null) ? false : left.Equals(right);
         }
 
-        public static bool operator !=(Assembly left, Assembly right)
+        public static bool operator !=(Assembly? left, Assembly? right)
         {
             return !(left == right);
         }
 
-        public static string CreateQualifiedName(string assemblyName, string typeName) => typeName + ", " + assemblyName;
+        public static string CreateQualifiedName(string? assemblyName, string? typeName) => typeName + ", " + assemblyName;
 
-        public static Assembly GetAssembly(Type type)
+        public static Assembly? GetAssembly(Type type)
         {
             if (type == null)
                 throw new ArgumentNullException(nameof(type));
@@ -202,7 +203,7 @@ public static Assembly GetAssembly(Type type)
         // Loads the assembly with a COFF based IMAGE containing
         // an emitted assembly. The assembly is loaded into a fully isolated ALC with resolution fully deferred to the AssemblyLoadContext.Default.
         // The second parameter is the raw bytes representing the symbol store that matches the assembly.
-        public static Assembly Load(byte[] rawAssembly, byte[] rawSymbolStore)
+        public static Assembly Load(byte[] rawAssembly, byte[]? rawSymbolStore)
         {
             if (rawAssembly == null)
                 throw new ArgumentNullException(nameof(rawAssembly));
@@ -252,9 +253,9 @@ public static Assembly LoadFile(string path)
             return result;
         }
 
-        private static Assembly LoadFromResolveHandler(object sender, ResolveEventArgs args)
+        private static Assembly? LoadFromResolveHandler(object? sender, ResolveEventArgs args)
         {
-            Assembly requestingAssembly = args.RequestingAssembly;
+            Assembly? requestingAssembly = args.RequestingAssembly;
             if (requestingAssembly == null)
             {
                 return null;
@@ -281,8 +282,8 @@ private static Assembly LoadFromResolveHandler(object sender, ResolveEventArgs a
             // Requestor assembly was loaded using loadFrom, so look for its dependencies
             // in the same folder as it.
             // Form the name of the assembly using the path of the assembly that requested its load.
-            AssemblyName requestedAssemblyName = new AssemblyName(args.Name);
-            string requestedAssemblyPath = Path.Combine(Path.GetDirectoryName(requestorPath), requestedAssemblyName.Name + ".dll");
+            AssemblyName requestedAssemblyName = new AssemblyName(args.Name!);
+            string requestedAssemblyPath = Path.Combine(Path.GetDirectoryName(requestorPath)!, requestedAssemblyName.Name + ".dll");
 
             try
             {
@@ -309,7 +310,7 @@ public static Assembly LoadFrom(string assemblyFile)
                 {
                     if (!s_loadFromHandlerSet)
                     {
-                        AssemblyLoadContext.AssemblyResolve += LoadFromResolveHandler;
+                        AssemblyLoadContext.AssemblyResolve += LoadFromResolveHandler!;
                         s_loadFromHandlerSet = true;
                     }
                 }
@@ -328,19 +329,19 @@ public static Assembly LoadFrom(string assemblyFile)
             return AssemblyLoadContext.Default.LoadFromAssemblyPath(fullPath);
         }
 
-        public static Assembly LoadFrom(string assemblyFile, byte[] hashValue, AssemblyHashAlgorithm hashAlgorithm)
+        public static Assembly LoadFrom(string? assemblyFile, byte[]? hashValue, AssemblyHashAlgorithm hashAlgorithm)
         {
             throw new NotSupportedException(SR.NotSupported_AssemblyLoadFromHash);
         }
 
         public static Assembly UnsafeLoadFrom(string assemblyFile) => LoadFrom(assemblyFile);
 
-        public Module LoadModule(string moduleName, byte[] rawModule) => LoadModule(moduleName, rawModule, null);
-        public virtual Module LoadModule(string moduleName, byte[] rawModule, byte[] rawSymbolStore) { throw NotImplemented.ByDesign; }
+        public Module LoadModule(string? moduleName, byte[]? rawModule) => LoadModule(moduleName, rawModule, null);
+        public virtual Module LoadModule(string? moduleName, byte[]? rawModule, byte[]? rawSymbolStore) { throw NotImplemented.ByDesign; }
 
-        public static Assembly ReflectionOnlyLoad(byte[] rawAssembly) { throw new PlatformNotSupportedException(SR.PlatformNotSupported_ReflectionOnly); }
-        public static Assembly ReflectionOnlyLoad(string assemblyString) { throw new PlatformNotSupportedException(SR.PlatformNotSupported_ReflectionOnly); }
-        public static Assembly ReflectionOnlyLoadFrom(string assemblyFile) { throw new PlatformNotSupportedException(SR.PlatformNotSupported_ReflectionOnly); }
+        public static Assembly ReflectionOnlyLoad(byte[]? rawAssembly) { throw new PlatformNotSupportedException(SR.PlatformNotSupported_ReflectionOnly); }
+        public static Assembly ReflectionOnlyLoad(string? assemblyString) { throw new PlatformNotSupportedException(SR.PlatformNotSupported_ReflectionOnly); }
+        public static Assembly ReflectionOnlyLoadFrom(string? assemblyFile) { throw new PlatformNotSupportedException(SR.PlatformNotSupported_ReflectionOnly); }
 
         public virtual SecurityRuleSet SecurityRuleSet => SecurityRuleSet.None;
     }
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/AssemblyDefaultAliasAttribute.cs b/src/System.Private.CoreLib/shared/System/Reflection/AssemblyDefaultAliasAttribute.cs
index ced35ed3fd5..6a8e7c0aa63 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/AssemblyDefaultAliasAttribute.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/AssemblyDefaultAliasAttribute.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 namespace System.Reflection
 {
     [AttributeUsage(AttributeTargets.Assembly, Inherited = false)]
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/AssemblyNameFormatter.cs b/src/System.Private.CoreLib/shared/System/Reflection/AssemblyNameFormatter.cs
index 90ccd1fe543..063dfc7a4bf 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/AssemblyNameFormatter.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/AssemblyNameFormatter.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.IO;
 using System.Text;
 using System.Globalization;
@@ -11,7 +12,7 @@ namespace System.Reflection
 {
     internal static class AssemblyNameFormatter
     {
-        public static string ComputeDisplayName(string name, Version version, string cultureName, byte[] pkt, AssemblyNameFlags flags, AssemblyContentType contentType)
+        public static string ComputeDisplayName(string? name, Version? version, string? cultureName, byte[]? pkt, AssemblyNameFlags flags, AssemblyContentType contentType)
         {
             const int PUBLIC_KEY_TOKEN_LEN = 8;
 
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/Binder.cs b/src/System.Private.CoreLib/shared/System/Reflection/Binder.cs
index 3dc5665d520..03ff487c5b0 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/Binder.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/Binder.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Globalization;
 
 namespace System.Reflection
@@ -9,11 +10,11 @@ namespace System.Reflection
     public abstract class Binder
     {
         protected Binder() { }
-        public abstract FieldInfo BindToField(BindingFlags bindingAttr, FieldInfo[] match, object value, CultureInfo culture);
-        public abstract MethodBase BindToMethod(BindingFlags bindingAttr, MethodBase[] match, ref object[] args, ParameterModifier[] modifiers, CultureInfo culture, string[] names, out object state);
-        public abstract object ChangeType(object value, Type type, CultureInfo culture);
+        public abstract FieldInfo BindToField(BindingFlags bindingAttr, FieldInfo[] match, object value, CultureInfo? culture);
+        public abstract MethodBase BindToMethod(BindingFlags bindingAttr, MethodBase[] match, ref object?[] args, ParameterModifier[]? modifiers, CultureInfo? culture, string[]? names, out object? state);
+        public abstract object ChangeType(object value, Type type, CultureInfo? culture);
         public abstract void ReorderArgumentArray(ref object[] args, object state);
-        public abstract MethodBase SelectMethod(BindingFlags bindingAttr, MethodBase[] match, Type[] types, ParameterModifier[] modifiers);
-        public abstract PropertyInfo SelectProperty(BindingFlags bindingAttr, PropertyInfo[] match, Type returnType, Type[] indexes, ParameterModifier[] modifiers);
+        public abstract MethodBase? SelectMethod(BindingFlags bindingAttr, MethodBase[] match, Type[] types, ParameterModifier[]? modifiers);
+        public abstract PropertyInfo? SelectProperty(BindingFlags bindingAttr, PropertyInfo[] match, Type? returnType, Type[]? indexes, ParameterModifier[]? modifiers);
     }
 }
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/ConstructorInfo.cs b/src/System.Private.CoreLib/shared/System/Reflection/ConstructorInfo.cs
index 1e31194cfca..57bbe3326f2 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/ConstructorInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/ConstructorInfo.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Diagnostics;
 using System.Globalization;
 using System.Runtime.CompilerServices;
@@ -16,14 +17,14 @@ protected ConstructorInfo() { }
 
         [DebuggerHidden]
         [DebuggerStepThrough]
-        public object Invoke(object[] parameters) => Invoke(BindingFlags.Default, binder: null, parameters: parameters, culture: null);
-        public abstract object Invoke(BindingFlags invokeAttr, Binder binder, object[] parameters, CultureInfo culture);
+        public object Invoke(object?[]? parameters) => Invoke(BindingFlags.Default, binder: null, parameters: parameters, culture: null);
+        public abstract object Invoke(BindingFlags invokeAttr, Binder? binder, object?[]? parameters, CultureInfo? culture);
 
-        public override bool Equals(object obj) => base.Equals(obj);
+        public override bool Equals(object? obj) => base.Equals(obj);
         public override int GetHashCode() => base.GetHashCode();
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static bool operator ==(ConstructorInfo left, ConstructorInfo right)
+        public static bool operator ==(ConstructorInfo? left, ConstructorInfo? right)
         {
             // Test "right" first to allow branch elimination when inlined for null checks (== null)
             // so it can become a simple test
@@ -34,7 +35,7 @@ protected ConstructorInfo() { }
             }
 
             // Try fast reference equality and opposite null check prior to calling the slower virtual Equals
-            if ((object)left == (object)right)
+            if ((object?)left == (object)right)
             {
                 return true;
             }
@@ -42,7 +43,7 @@ protected ConstructorInfo() { }
             return (left is null) ? false : left.Equals(right);
         }
 
-        public static bool operator !=(ConstructorInfo left, ConstructorInfo right) => !(left == right);
+        public static bool operator !=(ConstructorInfo? left, ConstructorInfo? right) => !(left == right);
 
         public static readonly string ConstructorName = ".ctor";
         public static readonly string TypeConstructorName = ".cctor";
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/EventInfo.cs b/src/System.Private.CoreLib/shared/System/Reflection/EventInfo.cs
index fa7e49df24b..f4e689d7e8d 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/EventInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/EventInfo.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Diagnostics;
 using System.Runtime.CompilerServices;
 
@@ -23,33 +24,33 @@ protected EventInfo() { }
         public MethodInfo[] GetOtherMethods() => GetOtherMethods(nonPublic: false);
         public virtual MethodInfo[] GetOtherMethods(bool nonPublic) { throw NotImplemented.ByDesign; }
 
-        public virtual MethodInfo AddMethod => GetAddMethod(nonPublic: true);
-        public virtual MethodInfo RemoveMethod => GetRemoveMethod(nonPublic: true);
-        public virtual MethodInfo RaiseMethod => GetRaiseMethod(nonPublic: true);
+        public virtual MethodInfo? AddMethod => GetAddMethod(nonPublic: true);
+        public virtual MethodInfo? RemoveMethod => GetRemoveMethod(nonPublic: true);
+        public virtual MethodInfo? RaiseMethod => GetRaiseMethod(nonPublic: true);
 
-        public MethodInfo GetAddMethod() => GetAddMethod(nonPublic: false);
-        public MethodInfo GetRemoveMethod() => GetRemoveMethod(nonPublic: false);
-        public MethodInfo GetRaiseMethod() => GetRaiseMethod(nonPublic: false);
+        public MethodInfo? GetAddMethod() => GetAddMethod(nonPublic: false);
+        public MethodInfo? GetRemoveMethod() => GetRemoveMethod(nonPublic: false);
+        public MethodInfo? GetRaiseMethod() => GetRaiseMethod(nonPublic: false);
 
-        public abstract MethodInfo GetAddMethod(bool nonPublic);
-        public abstract MethodInfo GetRemoveMethod(bool nonPublic);
-        public abstract MethodInfo GetRaiseMethod(bool nonPublic);
+        public abstract MethodInfo? GetAddMethod(bool nonPublic);
+        public abstract MethodInfo? GetRemoveMethod(bool nonPublic);
+        public abstract MethodInfo? GetRaiseMethod(bool nonPublic);
 
         public virtual bool IsMulticast
         {
             get
             {
-                Type cl = EventHandlerType;
+                Type? cl = EventHandlerType;
                 Type mc = typeof(MulticastDelegate);
                 return mc.IsAssignableFrom(cl);
             }
         }
 
-        public virtual Type EventHandlerType
+        public virtual Type? EventHandlerType
         {
             get
             {
-                MethodInfo m = GetAddMethod(true);
+                MethodInfo m = GetAddMethod(true)!;
                 ParameterInfo[] p = m.GetParametersNoCopy();
                 Type del = typeof(Delegate);
                 for (int i = 0; i < p.Length; i++)
@@ -64,9 +65,9 @@ public virtual Type EventHandlerType
 
         [DebuggerHidden]
         [DebuggerStepThrough]
-        public virtual void AddEventHandler(object target, Delegate handler)
+        public virtual void AddEventHandler(object? target, Delegate? handler)
         {
-            MethodInfo addMethod = GetAddMethod(nonPublic: false);
+            MethodInfo? addMethod = GetAddMethod(nonPublic: false);
 
             if (addMethod == null)
                 throw new InvalidOperationException(SR.InvalidOperation_NoPublicAddMethod);
@@ -76,14 +77,14 @@ public virtual void AddEventHandler(object target, Delegate handler)
                 throw new InvalidOperationException(SR.InvalidOperation_NotSupportedOnWinRTEvent);
 #endif //#if FEATURE_COMINTEROP
 
-            addMethod.Invoke(target, new object[] { handler });
+            addMethod.Invoke(target, new object?[] { handler });
         }
 
         [DebuggerHidden]
         [DebuggerStepThrough]
-        public virtual void RemoveEventHandler(object target, Delegate handler)
+        public virtual void RemoveEventHandler(object? target, Delegate? handler)
         {
-            MethodInfo removeMethod = GetRemoveMethod(nonPublic: false);
+            MethodInfo? removeMethod = GetRemoveMethod(nonPublic: false);
 
             if (removeMethod == null)
                 throw new InvalidOperationException(SR.InvalidOperation_NoPublicRemoveMethod);
@@ -94,14 +95,14 @@ public virtual void RemoveEventHandler(object target, Delegate handler)
                 throw new InvalidOperationException(SR.InvalidOperation_NotSupportedOnWinRTEvent);
 #endif //#if FEATURE_COMINTEROP
 
-            removeMethod.Invoke(target, new object[] { handler });
+            removeMethod.Invoke(target, new object?[] { handler });
         }
 
-        public override bool Equals(object obj) => base.Equals(obj);
+        public override bool Equals(object? obj) => base.Equals(obj);
         public override int GetHashCode() => base.GetHashCode();
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static bool operator ==(EventInfo left, EventInfo right)
+        public static bool operator ==(EventInfo? left, EventInfo? right)
         {
             // Test "right" first to allow branch elimination when inlined for null checks (== null)
             // so it can become a simple test
@@ -112,7 +113,7 @@ public virtual void RemoveEventHandler(object target, Delegate handler)
             }
 
             // Try fast reference equality and opposite null check prior to calling the slower virtual Equals
-            if ((object)left == (object)right)
+            if ((object?)left == (object)right)
             {
                 return true;
             }
@@ -120,6 +121,6 @@ public virtual void RemoveEventHandler(object target, Delegate handler)
             return (left is null) ? false : left.Equals(right);
         }
 
-        public static bool operator !=(EventInfo left, EventInfo right) => !(left == right);
+        public static bool operator !=(EventInfo? left, EventInfo? right) => !(left == right);
     }
 }
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/FieldInfo.cs b/src/System.Private.CoreLib/shared/System/Reflection/FieldInfo.cs
index fd577b668dc..a4875adb48b 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/FieldInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/FieldInfo.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Diagnostics;
 using System.Globalization;
 using System.Runtime.CompilerServices;
@@ -37,11 +38,11 @@ protected FieldInfo() { }
 
         public abstract RuntimeFieldHandle FieldHandle { get; }
 
-        public override bool Equals(object obj) => base.Equals(obj);
+        public override bool Equals(object? obj) => base.Equals(obj);
         public override int GetHashCode() => base.GetHashCode();
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static bool operator ==(FieldInfo left, FieldInfo right)
+        public static bool operator ==(FieldInfo? left, FieldInfo? right)
         {
             // Test "right" first to allow branch elimination when inlined for null checks (== null)
             // so it can become a simple test
@@ -52,7 +53,7 @@ protected FieldInfo() { }
             }
 
             // Try fast reference equality and opposite null check prior to calling the slower virtual Equals
-            if ((object)left == (object)right)
+            if ((object?)left == (object)right)
             {
                 return true;
             }
@@ -60,14 +61,14 @@ protected FieldInfo() { }
             return (left is null) ? false : left.Equals(right);
         }
 
-        public static bool operator !=(FieldInfo left, FieldInfo right) => !(left == right);
+        public static bool operator !=(FieldInfo? left, FieldInfo? right) => !(left == right);
 
-        public abstract object GetValue(object obj);
+        public abstract object? GetValue(object? obj);
 
         [DebuggerHidden]
         [DebuggerStepThrough]
-        public void SetValue(object obj, object value) => SetValue(obj, value, BindingFlags.Default, Type.DefaultBinder, null);
-        public abstract void SetValue(object obj, object value, BindingFlags invokeAttr, Binder binder, CultureInfo culture);
+        public void SetValue(object? obj, object value) => SetValue(obj, value, BindingFlags.Default, Type.DefaultBinder, null);
+        public abstract void SetValue(object? obj, object? value, BindingFlags invokeAttr, Binder? binder, CultureInfo? culture);
 
         [CLSCompliant(false)]
         public virtual void SetValueDirect(TypedReference obj, object value) { throw new NotSupportedException(SR.NotSupported_AbstractNonCLS); }
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/ICustomAttributeProvider.cs b/src/System.Private.CoreLib/shared/System/Reflection/ICustomAttributeProvider.cs
index 3cae295bc49..660765cd8cf 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/ICustomAttributeProvider.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/ICustomAttributeProvider.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 namespace System.Reflection
 {
     public interface ICustomAttributeProvider
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/IReflect.cs b/src/System.Private.CoreLib/shared/System/Reflection/IReflect.cs
index a7e84b61680..5099f8bd39c 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/IReflect.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/IReflect.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Globalization;
 
 namespace System.Reflection
@@ -11,30 +12,30 @@ public interface IReflect
         // Return the requested method if it is implemented by the Reflection object.  The
         // match is based upon the name and DescriptorInfo which describes the signature
         // of the method. 
-        MethodInfo GetMethod(string name, BindingFlags bindingAttr, Binder binder, Type[] types, ParameterModifier[] modifiers);
+        MethodInfo? GetMethod(string name, BindingFlags bindingAttr, Binder? binder, Type[] types, ParameterModifier[]? modifiers);
 
         // Return the requested method if it is implemented by the Reflection object.  The
         // match is based upon the name of the method.  If the object implementes multiple methods
         // with the same name an AmbiguousMatchException is thrown.
-        MethodInfo GetMethod(string name, BindingFlags bindingAttr);
+        MethodInfo? GetMethod(string name, BindingFlags bindingAttr);
 
         MethodInfo[] GetMethods(BindingFlags bindingAttr);
 
         // Return the requestion field if it is implemented by the Reflection object.  The
         // match is based upon a name.  There cannot be more than a single field with
         // a name.
-        FieldInfo GetField(string name, BindingFlags bindingAttr);
+        FieldInfo? GetField(string name, BindingFlags bindingAttr);
 
         FieldInfo[] GetFields(BindingFlags bindingAttr);
 
         // Return the property based upon name.  If more than one property has the given
         // name an AmbiguousMatchException will be thrown.  Returns null if no property
         // is found.
-        PropertyInfo GetProperty(string name, BindingFlags bindingAttr);
+        PropertyInfo? GetProperty(string name, BindingFlags bindingAttr);
 
         // Return the property based upon the name and Descriptor info describing the property
         // indexing.  Return null if no property is found.
-        PropertyInfo GetProperty(string name, BindingFlags bindingAttr, Binder binder, Type returnType, Type[] types, ParameterModifier[] modifiers);
+        PropertyInfo? GetProperty(string name, BindingFlags bindingAttr, Binder? binder, Type? returnType, Type[] types, ParameterModifier[]? modifiers);
 
         // Returns an array of PropertyInfos for all the properties defined on 
         // the Reflection object.
@@ -67,7 +68,7 @@ public interface IReflect
         // For the default binder, the most specific method will be selected.
         // 
         // This will invoke a specific member...
-        object InvokeMember(string name, BindingFlags invokeAttr, Binder binder, object target, object[] args, ParameterModifier[] modifiers, CultureInfo culture, string[] namedParameters);
+        object? InvokeMember(string name, BindingFlags invokeAttr, Binder? binder, object? target, object[]? args, ParameterModifier[]? modifiers, CultureInfo? culture, string[]? namedParameters);
 
         // Return the underlying Type that represents the IReflect Object.  For expando object,
         // this is the (Object) IReflectInstance.GetType().  For Type object it is this.
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/IReflectableType.cs b/src/System.Private.CoreLib/shared/System/Reflection/IReflectableType.cs
index 5e2c0edab4b..0264f6321e2 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/IReflectableType.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/IReflectableType.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 namespace System.Reflection
 {
     public interface IReflectableType
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/IntrospectionExtensions.cs b/src/System.Private.CoreLib/shared/System/Reflection/IntrospectionExtensions.cs
index acf5987d448..cdf88360185 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/IntrospectionExtensions.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/IntrospectionExtensions.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Diagnostics;
 
 namespace System.Reflection
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/LocalVariableInfo.cs b/src/System.Private.CoreLib/shared/System/Reflection/LocalVariableInfo.cs
index 1540bde531d..1040d50a585 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/LocalVariableInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/LocalVariableInfo.cs
@@ -2,13 +2,14 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Diagnostics;
 
 namespace System.Reflection
 {
     public class LocalVariableInfo
     {
-        public virtual Type LocalType { get { Debug.Fail("type must be set!"); return null; } }
+        public virtual Type LocalType { get { Debug.Fail("type must be set!"); return null!; } }
         public virtual int LocalIndex => 0;
         public virtual bool IsPinned => false;
         protected LocalVariableInfo() { }
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/ManifestResourceInfo.cs b/src/System.Private.CoreLib/shared/System/Reflection/ManifestResourceInfo.cs
index b9c56ab857f..02296da7b9a 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/ManifestResourceInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/ManifestResourceInfo.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 namespace System.Reflection
 {
     public class ManifestResourceInfo
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/MemberFilter.cs b/src/System.Private.CoreLib/shared/System/Reflection/MemberFilter.cs
index bb1b15796aa..3f3282325ea 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/MemberFilter.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/MemberFilter.cs
@@ -2,7 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 namespace System.Reflection
 {
     public delegate bool MemberFilter(MemberInfo m, object filterCriteria);
-}
\ No newline at end of file
+}
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/MemberInfo.cs b/src/System.Private.CoreLib/shared/System/Reflection/MemberInfo.cs
index 8d15005fb47..adf7bbaa8b6 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/MemberInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/MemberInfo.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 
@@ -13,8 +14,8 @@ protected MemberInfo() { }
 
         public abstract MemberTypes MemberType { get; }
         public abstract string Name { get; }
-        public abstract Type DeclaringType { get; }
-        public abstract Type ReflectedType { get; }
+        public abstract Type? DeclaringType { get; }
+        public abstract Type? ReflectedType { get; }
 
         public virtual Module Module
         {
@@ -23,8 +24,7 @@ public virtual Module Module
                 // This check is necessary because for some reason, Type adds a new "Module" property that hides the inherited one instead 
                 // of overriding.
 
-                Type type = this as Type;
-                if (type != null)
+                if (this is Type type)
                     return type.Module;
 
                 throw NotImplemented.ByDesign;
@@ -42,11 +42,11 @@ public virtual Module Module
         public virtual bool IsCollectible => true;
         public virtual int MetadataToken { get { throw new InvalidOperationException(); } }
 
-        public override bool Equals(object obj) => base.Equals(obj);
+        public override bool Equals(object? obj) => base.Equals(obj);
         public override int GetHashCode() => base.GetHashCode();
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static bool operator ==(MemberInfo left, MemberInfo right)
+        public static bool operator ==(MemberInfo? left, MemberInfo? right)
         {
             // Test "right" first to allow branch elimination when inlined for null checks (== null)
             // so it can become a simple test
@@ -57,7 +57,7 @@ public virtual Module Module
             }
 
             // Try fast reference equality and opposite null check prior to calling the slower virtual Equals
-            if ((object)left == (object)right)
+            if ((object?)left == (object)right)
             {
                 return true;
             }
@@ -65,6 +65,6 @@ public virtual Module Module
             return (left is null) ? false : left.Equals(right);
         }
 
-        public static bool operator !=(MemberInfo left, MemberInfo right) => !(left == right);
+        public static bool operator !=(MemberInfo? left, MemberInfo? right) => !(left == right);
     }
 }
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/MethodBase.cs b/src/System.Private.CoreLib/shared/System/Reflection/MethodBase.cs
index 914689c5ba0..551b78222f5 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/MethodBase.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/MethodBase.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Globalization;
 using System.Diagnostics;
 using System.Runtime.CompilerServices;
@@ -51,8 +52,8 @@ public bool IsConstructor
 
         [DebuggerHidden]
         [DebuggerStepThrough]
-        public object Invoke(object obj, object[] parameters) => Invoke(obj, BindingFlags.Default, binder: null, parameters: parameters, culture: null);
-        public abstract object Invoke(object obj, BindingFlags invokeAttr, Binder binder, object[] parameters, CultureInfo culture);
+        public object? Invoke(object? obj, object?[]? parameters) => Invoke(obj, BindingFlags.Default, binder: null, parameters: parameters, culture: null);
+        public abstract object? Invoke(object? obj, BindingFlags invokeAttr, Binder? binder, object?[]? parameters, CultureInfo? culture);
 
         public abstract RuntimeMethodHandle MethodHandle { get; }
 
@@ -60,11 +61,11 @@ public bool IsConstructor
         public virtual bool IsSecuritySafeCritical { get { throw NotImplemented.ByDesign; } }
         public virtual bool IsSecurityTransparent { get { throw NotImplemented.ByDesign; } }
 
-        public override bool Equals(object obj) => base.Equals(obj);
+        public override bool Equals(object? obj) => base.Equals(obj);
         public override int GetHashCode() => base.GetHashCode();
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static bool operator ==(MethodBase left, MethodBase right)
+        public static bool operator ==(MethodBase? left, MethodBase? right)
         {
             // Test "right" first to allow branch elimination when inlined for null checks (== null)
             // so it can become a simple test
@@ -75,7 +76,7 @@ public bool IsConstructor
             }
 
             // Try fast reference equality and opposite null check prior to calling the slower virtual Equals
-            if ((object)left == (object)right)
+            if ((object?)left == (object)right)
             {
                 return true;
             }
@@ -83,6 +84,6 @@ public bool IsConstructor
             return (left is null) ? false : left.Equals(right);
         }
 
-        public static bool operator !=(MethodBase left, MethodBase right) => !(left == right);
+        public static bool operator !=(MethodBase? left, MethodBase? right) => !(left == right);
     }
 }
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/MethodBody.cs b/src/System.Private.CoreLib/shared/System/Reflection/MethodBody.cs
index bdf53ad12f6..745a9a0c60b 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/MethodBody.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/MethodBody.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Collections.Generic;
 
 namespace System.Reflection
@@ -13,7 +14,7 @@ protected MethodBody() { }
         public virtual IList<LocalVariableInfo> LocalVariables => throw new ArgumentNullException("array");
         public virtual int MaxStackSize => 0;
         public virtual bool InitLocals => false;
-        public virtual byte[] GetILAsByteArray() => null;
+        public virtual byte[]? GetILAsByteArray() => null;
         public virtual IList<ExceptionHandlingClause> ExceptionHandlingClauses => throw new ArgumentNullException("array");
     }
 }
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/MethodInfo.Internal.cs b/src/System.Private.CoreLib/shared/System/Reflection/MethodInfo.Internal.cs
index 2806be639ef..273f1639d5d 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/MethodInfo.Internal.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/MethodInfo.Internal.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 namespace System.Reflection
 {
     public abstract partial class MethodInfo : MethodBase
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/MethodInfo.cs b/src/System.Private.CoreLib/shared/System/Reflection/MethodInfo.cs
index 55528543410..260e1338da0 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/MethodInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/MethodInfo.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Runtime.CompilerServices;
 
 namespace System.Reflection
@@ -12,25 +13,25 @@ protected MethodInfo() { }
 
         public override MemberTypes MemberType => MemberTypes.Method;
 
-        public virtual ParameterInfo ReturnParameter { get { throw NotImplemented.ByDesign; } }
-        public virtual Type ReturnType { get { throw NotImplemented.ByDesign; } }
+        public virtual ParameterInfo? ReturnParameter { get { throw NotImplemented.ByDesign; } }
+        public virtual Type? ReturnType { get { throw NotImplemented.ByDesign; } }
 
         public override Type[] GetGenericArguments() { throw new NotSupportedException(SR.NotSupported_SubclassOverride); }
-        public virtual MethodInfo GetGenericMethodDefinition() { throw new NotSupportedException(SR.NotSupported_SubclassOverride); }
-        public virtual MethodInfo MakeGenericMethod(params Type[] typeArguments) { throw new NotSupportedException(SR.NotSupported_SubclassOverride); }
+        public virtual MethodInfo? GetGenericMethodDefinition() { throw new NotSupportedException(SR.NotSupported_SubclassOverride); }
+        public virtual MethodInfo? MakeGenericMethod(params Type[] typeArguments) { throw new NotSupportedException(SR.NotSupported_SubclassOverride); }
 
-        public abstract MethodInfo GetBaseDefinition();
+        public abstract MethodInfo? GetBaseDefinition();
 
-        public abstract ICustomAttributeProvider ReturnTypeCustomAttributes { get; }
+        public abstract ICustomAttributeProvider? ReturnTypeCustomAttributes { get; }
 
         public virtual Delegate CreateDelegate(Type delegateType) { throw new NotSupportedException(SR.NotSupported_SubclassOverride); }
-        public virtual Delegate CreateDelegate(Type delegateType, object target) { throw new NotSupportedException(SR.NotSupported_SubclassOverride); }
+        public virtual Delegate CreateDelegate(Type delegateType, object? target) { throw new NotSupportedException(SR.NotSupported_SubclassOverride); }
 
-        public override bool Equals(object obj) => base.Equals(obj);
+        public override bool Equals(object? obj) => base.Equals(obj);
         public override int GetHashCode() => base.GetHashCode();
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static bool operator ==(MethodInfo left, MethodInfo right)
+        public static bool operator ==(MethodInfo? left, MethodInfo? right)
         {
             // Test "right" first to allow branch elimination when inlined for null checks (== null)
             // so it can become a simple test
@@ -41,7 +42,7 @@ protected MethodInfo() { }
             }
 
             // Try fast reference equality and opposite null check prior to calling the slower virtual Equals
-            if ((object)left == (object)right)
+            if ((object?)left == (object)right)
             {
                 return true;
             }
@@ -49,6 +50,6 @@ protected MethodInfo() { }
             return (left is null) ? false : left.Equals(right);
         }
 
-        public static bool operator !=(MethodInfo left, MethodInfo right) => !(left == right);
+        public static bool operator !=(MethodInfo? left, MethodInfo? right) => !(left == right);
     }
 }
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/Missing.cs b/src/System.Private.CoreLib/shared/System/Reflection/Missing.cs
index 46ab32fccf6..7f994f9895d 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/Missing.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/Missing.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Runtime.Serialization;
 
 namespace System.Reflection
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/Module.cs b/src/System.Private.CoreLib/shared/System/Reflection/Module.cs
index 8042160c32d..b54e7070966 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/Module.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/Module.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 using System.Runtime.Serialization;
@@ -30,7 +31,7 @@ protected Module() { }
         public virtual object[] GetCustomAttributes(bool inherit) { throw NotImplemented.ByDesign; }
         public virtual object[] GetCustomAttributes(Type attributeType, bool inherit) { throw NotImplemented.ByDesign; }
 
-        public MethodInfo GetMethod(string name)
+        public MethodInfo? GetMethod(string name)
         {
             if (name == null)
                 throw new ArgumentNullException(nameof(name));
@@ -38,8 +39,8 @@ public MethodInfo GetMethod(string name)
             return GetMethodImpl(name, Module.DefaultLookup, null, CallingConventions.Any, null, null);
         }
 
-        public MethodInfo GetMethod(string name, Type[] types) => GetMethod(name, Module.DefaultLookup, null, CallingConventions.Any, types, null);
-        public MethodInfo GetMethod(string name, BindingFlags bindingAttr, Binder binder, CallingConventions callConvention, Type[] types, ParameterModifier[] modifiers)
+        public MethodInfo? GetMethod(string name, Type[] types) => GetMethod(name, Module.DefaultLookup, null, CallingConventions.Any, types, null);
+        public MethodInfo? GetMethod(string name, BindingFlags bindingAttr, Binder? binder, CallingConventions callConvention, Type[] types, ParameterModifier[]? modifiers)
         {
             if (name == null)
                 throw new ArgumentNullException(nameof(name));
@@ -53,31 +54,31 @@ public MethodInfo GetMethod(string name, BindingFlags bindingAttr, Binder binder
             return GetMethodImpl(name, bindingAttr, binder, callConvention, types, modifiers);
         }
 
-        protected virtual MethodInfo GetMethodImpl(string name, BindingFlags bindingAttr, Binder binder, CallingConventions callConvention, Type[] types, ParameterModifier[] modifiers) { throw NotImplemented.ByDesign; }
+        protected virtual MethodInfo? GetMethodImpl(string name, BindingFlags bindingAttr, Binder? binder, CallingConventions callConvention, Type[]? types, ParameterModifier[]? modifiers) { throw NotImplemented.ByDesign; }
 
         public MethodInfo[] GetMethods() => GetMethods(Module.DefaultLookup);
         public virtual MethodInfo[] GetMethods(BindingFlags bindingFlags) { throw NotImplemented.ByDesign; }
 
-        public FieldInfo GetField(string name) => GetField(name, Module.DefaultLookup);
-        public virtual FieldInfo GetField(string name, BindingFlags bindingAttr) { throw NotImplemented.ByDesign; }
+        public FieldInfo? GetField(string name) => GetField(name, Module.DefaultLookup);
+        public virtual FieldInfo? GetField(string name, BindingFlags bindingAttr) { throw NotImplemented.ByDesign; }
 
         public FieldInfo[] GetFields() => GetFields(Module.DefaultLookup);
         public virtual FieldInfo[] GetFields(BindingFlags bindingFlags) { throw NotImplemented.ByDesign; }
 
         public virtual Type[] GetTypes() { throw NotImplemented.ByDesign; }
 
-        public virtual Type GetType(string className) => GetType(className, throwOnError: false, ignoreCase: false);
-        public virtual Type GetType(string className, bool ignoreCase) => GetType(className, throwOnError: false, ignoreCase: ignoreCase);
-        public virtual Type GetType(string className, bool throwOnError, bool ignoreCase) { throw NotImplemented.ByDesign; }
+        public virtual Type? GetType(string className) => GetType(className, throwOnError: false, ignoreCase: false);
+        public virtual Type? GetType(string className, bool ignoreCase) => GetType(className, throwOnError: false, ignoreCase: ignoreCase);
+        public virtual Type? GetType(string className, bool throwOnError, bool ignoreCase) { throw NotImplemented.ByDesign; }
 
-        public virtual Type[] FindTypes(TypeFilter filter, object filterCriteria)
+        public virtual Type[] FindTypes(TypeFilter? filter, object filterCriteria)
         {
             Type[] c = GetTypes();
             int cnt = 0;
             for (int i = 0; i < c.Length; i++)
             {
                 if (filter != null && !filter(c[i], filterCriteria))
-                    c[i] = null;
+                    c[i] = null!;
                 else
                     cnt++;
             }
@@ -96,28 +97,28 @@ public virtual Type[] FindTypes(TypeFilter filter, object filterCriteria)
 
         public virtual int MetadataToken { get { throw NotImplemented.ByDesign; } }
 
-        public FieldInfo ResolveField(int metadataToken) => ResolveField(metadataToken, null, null);
-        public virtual FieldInfo ResolveField(int metadataToken, Type[] genericTypeArguments, Type[] genericMethodArguments) { throw NotImplemented.ByDesign; }
+        public FieldInfo? ResolveField(int metadataToken) => ResolveField(metadataToken, null, null);
+        public virtual FieldInfo? ResolveField(int metadataToken, Type[]? genericTypeArguments, Type[]? genericMethodArguments) { throw NotImplemented.ByDesign; }
 
-        public MemberInfo ResolveMember(int metadataToken) => ResolveMember(metadataToken, null, null);
-        public virtual MemberInfo ResolveMember(int metadataToken, Type[] genericTypeArguments, Type[] genericMethodArguments) { throw NotImplemented.ByDesign; }
+        public MemberInfo? ResolveMember(int metadataToken) => ResolveMember(metadataToken, null, null);
+        public virtual MemberInfo? ResolveMember(int metadataToken, Type[]? genericTypeArguments, Type[]? genericMethodArguments) { throw NotImplemented.ByDesign; }
 
-        public MethodBase ResolveMethod(int metadataToken) => ResolveMethod(metadataToken, null, null);
-        public virtual MethodBase ResolveMethod(int metadataToken, Type[] genericTypeArguments, Type[] genericMethodArguments) { throw NotImplemented.ByDesign; }
+        public MethodBase? ResolveMethod(int metadataToken) => ResolveMethod(metadataToken, null, null);
+        public virtual MethodBase? ResolveMethod(int metadataToken, Type[]? genericTypeArguments, Type[]? genericMethodArguments) { throw NotImplemented.ByDesign; }
 
         public virtual byte[] ResolveSignature(int metadataToken) { throw NotImplemented.ByDesign; }
         public virtual string ResolveString(int metadataToken) { throw NotImplemented.ByDesign; }
 
         public Type ResolveType(int metadataToken) => ResolveType(metadataToken, null, null);
-        public virtual Type ResolveType(int metadataToken, Type[] genericTypeArguments, Type[] genericMethodArguments) { throw NotImplemented.ByDesign; }
+        public virtual Type ResolveType(int metadataToken, Type[]? genericTypeArguments, Type[]? genericMethodArguments) { throw NotImplemented.ByDesign; }
 
         public virtual void GetObjectData(SerializationInfo info, StreamingContext context) { throw NotImplemented.ByDesign; }
 
-        public override bool Equals(object o) => base.Equals(o);
+        public override bool Equals(object? o) => base.Equals(o);
         public override int GetHashCode() => base.GetHashCode();
         
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static bool operator ==(Module left, Module right)
+        public static bool operator ==(Module? left, Module? right)
         {
             // Test "right" first to allow branch elimination when inlined for null checks (== null)
             // so it can become a simple test
@@ -128,7 +129,7 @@ public virtual Type[] FindTypes(TypeFilter filter, object filterCriteria)
             }
 
             // Try fast reference equality and opposite null check prior to calling the slower virtual Equals
-            if ((object)left == (object)right)
+            if ((object?)left == (object)right)
             {
                 return true;
             }
@@ -136,11 +137,11 @@ public virtual Type[] FindTypes(TypeFilter filter, object filterCriteria)
             return (left is null) ? false : left.Equals(right);
         }
 
-        public static bool operator !=(Module left, Module right) => !(left == right);
+        public static bool operator !=(Module? left, Module? right) => !(left == right);
 
         public override string ToString() => ScopeName;
 
-        public static readonly TypeFilter FilterTypeName = (m, c) => FilterTypeNameImpl(m, c, StringComparison.Ordinal);
+        public static readonly TypeFilter FilterTypeName = (m, c) => FilterTypeNameImpl(m, c, StringComparison.Ordinal); // TODO-NULLABLE https://github.com/dotnet/roslyn/issues/23268
         public static readonly TypeFilter FilterTypeNameIgnoreCase = (m, c) => FilterTypeNameImpl(m, c, StringComparison.OrdinalIgnoreCase);
 
         private const BindingFlags DefaultLookup = BindingFlags.Instance | BindingFlags.Static | BindingFlags.Public;
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/ModuleResolveEventHandler.cs b/src/System.Private.CoreLib/shared/System/Reflection/ModuleResolveEventHandler.cs
index eb8926b5db5..6c52a222149 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/ModuleResolveEventHandler.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/ModuleResolveEventHandler.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 namespace System.Reflection
 {
     public delegate Module ModuleResolveEventHandler(object sender, ResolveEventArgs e);
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/ParameterInfo.cs b/src/System.Private.CoreLib/shared/System/Reflection/ParameterInfo.cs
index 94bfffaa538..3db4c5c1b9c 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/ParameterInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/ParameterInfo.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Collections.Generic;
 using System.Runtime.Serialization;
 
@@ -13,8 +14,8 @@ protected ParameterInfo() { }
 
         public virtual ParameterAttributes Attributes => AttrsImpl;
         public virtual MemberInfo Member => MemberImpl;
-        public virtual string Name => NameImpl;
-        public virtual Type ParameterType => ClassImpl;
+        public virtual string? Name => NameImpl;
+        public virtual Type ParameterType => ClassImpl!;
         public virtual int Position => PositionImpl;
 
         public bool IsIn => (Attributes & ParameterAttributes.In) != 0;
@@ -23,8 +24,8 @@ protected ParameterInfo() { }
         public bool IsOut => (Attributes & ParameterAttributes.Out) != 0;
         public bool IsRetval => (Attributes & ParameterAttributes.Retval) != 0;
 
-        public virtual object DefaultValue { get { throw NotImplemented.ByDesign; } }
-        public virtual object RawDefaultValue { get { throw NotImplemented.ByDesign; } }
+        public virtual object? DefaultValue { get { throw NotImplemented.ByDesign; } }
+        public virtual object? RawDefaultValue { get { throw NotImplemented.ByDesign; } }
         public virtual bool HasDefaultValue { get { throw NotImplemented.ByDesign; } }
 
         public virtual bool IsDefined(Type attributeType, bool inherit)
@@ -60,7 +61,7 @@ public object GetRealObject(StreamingContext context)
             if (MemberImpl == null)
                 throw new SerializationException(SR.Serialization_InsufficientState);
 
-            ParameterInfo[] args = null;
+            ParameterInfo[]? args = null;
 
             switch (MemberImpl.MemberType)
             {
@@ -69,7 +70,7 @@ public object GetRealObject(StreamingContext context)
                     if (PositionImpl == -1)
                     {
                         if (MemberImpl.MemberType == MemberTypes.Method)
-                            return ((MethodInfo)MemberImpl).ReturnParameter;
+                            return ((MethodInfo)MemberImpl).ReturnParameter!;
                         else
                             throw new SerializationException(SR.Serialization_BadParameterInfo);
                     }
@@ -99,10 +100,10 @@ public object GetRealObject(StreamingContext context)
         public override string ToString() => ParameterType.FormatTypeName() + " " + Name;
 
         protected ParameterAttributes AttrsImpl;
-        protected Type ClassImpl;
-        protected object DefaultValueImpl;
-        protected MemberInfo MemberImpl;
-        protected string NameImpl;
+        protected Type? ClassImpl;
+        protected object? DefaultValueImpl;
+        protected MemberInfo MemberImpl = null!;
+        protected string? NameImpl;
         protected int PositionImpl;
 
         private const int MetadataToken_ParamDef = 0x08000000;
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/Pointer.cs b/src/System.Private.CoreLib/shared/System/Reflection/Pointer.cs
index e1a9990cf0e..5761eabb443 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/Pointer.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/Pointer.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Diagnostics;
 using System.Runtime.Serialization;
 
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/PropertyInfo.cs b/src/System.Private.CoreLib/shared/System/Reflection/PropertyInfo.cs
index 7fbb7eed444..6edfe7159c0 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/PropertyInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/PropertyInfo.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Diagnostics;
 using System.Globalization;
 using System.Runtime.CompilerServices;
@@ -26,41 +27,41 @@ protected PropertyInfo() { }
         public MethodInfo[] GetAccessors() => GetAccessors(nonPublic: false);
         public abstract MethodInfo[] GetAccessors(bool nonPublic);
 
-        public virtual MethodInfo GetMethod => GetGetMethod(nonPublic: true);
-        public MethodInfo GetGetMethod() => GetGetMethod(nonPublic: false);
-        public abstract MethodInfo GetGetMethod(bool nonPublic);
+        public virtual MethodInfo? GetMethod => GetGetMethod(nonPublic: true);
+        public MethodInfo? GetGetMethod() => GetGetMethod(nonPublic: false);
+        public abstract MethodInfo? GetGetMethod(bool nonPublic);
 
-        public virtual MethodInfo SetMethod => GetSetMethod(nonPublic: true);
-        public MethodInfo GetSetMethod() => GetSetMethod(nonPublic: false);
-        public abstract MethodInfo GetSetMethod(bool nonPublic);
+        public virtual MethodInfo? SetMethod => GetSetMethod(nonPublic: true);
+        public MethodInfo? GetSetMethod() => GetSetMethod(nonPublic: false);
+        public abstract MethodInfo? GetSetMethod(bool nonPublic);
 
         public virtual Type[] GetOptionalCustomModifiers() => Array.Empty<Type>();
         public virtual Type[] GetRequiredCustomModifiers() => Array.Empty<Type>();
 
         [DebuggerHidden]
         [DebuggerStepThrough]
-        public object GetValue(object obj) => GetValue(obj, index: null);
+        public object? GetValue(object? obj) => GetValue(obj, index: null);
         [DebuggerHidden]
         [DebuggerStepThrough]
-        public virtual object GetValue(object obj, object[] index) => GetValue(obj, BindingFlags.Default, binder: null, index: index, culture: null);
-        public abstract object GetValue(object obj, BindingFlags invokeAttr, Binder binder, object[] index, CultureInfo culture);
+        public virtual object? GetValue(object? obj, object?[]? index) => GetValue(obj, BindingFlags.Default, binder: null, index: index, culture: null);
+        public abstract object? GetValue(object? obj, BindingFlags invokeAttr, Binder? binder, object?[]? index, CultureInfo? culture);
 
         public virtual object GetConstantValue() { throw NotImplemented.ByDesign; }
         public virtual object GetRawConstantValue() { throw NotImplemented.ByDesign; }
 
         [DebuggerHidden]
         [DebuggerStepThrough]
-        public void SetValue(object obj, object value) => SetValue(obj, value, index: null);
+        public void SetValue(object? obj, object? value) => SetValue(obj, value, index: null);
         [DebuggerHidden]
         [DebuggerStepThrough]
-        public virtual void SetValue(object obj, object value, object[] index) => SetValue(obj, value, BindingFlags.Default, binder: null, index: index, culture: null);
-        public abstract void SetValue(object obj, object value, BindingFlags invokeAttr, Binder binder, object[] index, CultureInfo culture);
+        public virtual void SetValue(object? obj, object? value, object[]? index) => SetValue(obj, value, BindingFlags.Default, binder: null, index: index, culture: null);
+        public abstract void SetValue(object? obj, object? value, BindingFlags invokeAttr, Binder? binder, object[]? index, CultureInfo? culture);
 
-        public override bool Equals(object obj) => base.Equals(obj);
+        public override bool Equals(object? obj) => base.Equals(obj);
         public override int GetHashCode() => base.GetHashCode();
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static bool operator ==(PropertyInfo left, PropertyInfo right)
+        public static bool operator ==(PropertyInfo? left, PropertyInfo? right)
         {
             // Test "right" first to allow branch elimination when inlined for null checks (== null)
             // so it can become a simple test
@@ -71,7 +72,7 @@ protected PropertyInfo() { }
             }
 
             // Try fast reference equality and opposite null check prior to calling the slower virtual Equals
-            if ((object)left == (object)right)
+            if ((object?)left == (object)right)
             {
                 return true;
             }
@@ -79,6 +80,6 @@ protected PropertyInfo() { }
             return (left is null) ? false : left.Equals(right);
         }
 
-        public static bool operator !=(PropertyInfo left, PropertyInfo right) => !(left == right);
+        public static bool operator !=(PropertyInfo? left, PropertyInfo? right) => !(left == right);
     }
 }
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/ReflectionContext.cs b/src/System.Private.CoreLib/shared/System/Reflection/ReflectionContext.cs
index e9e93dab81b..f7f9ce4f80e 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/ReflectionContext.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/ReflectionContext.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 namespace System.Reflection
 {
     public abstract class ReflectionContext
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/SignatureArrayType.cs b/src/System.Private.CoreLib/shared/System/Reflection/SignatureArrayType.cs
index 52011b8a9d1..6a4b8f076b4 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/SignatureArrayType.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/SignatureArrayType.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Diagnostics;
 
 namespace System.Reflection
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/SignatureByRefType.cs b/src/System.Private.CoreLib/shared/System/Reflection/SignatureByRefType.cs
index eb5f6de42ea..e0d0a3a4782 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/SignatureByRefType.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/SignatureByRefType.cs
@@ -2,8 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using System;
-
+#nullable enable
 namespace System.Reflection
 {
     internal sealed class SignatureByRefType : SignatureHasElementType
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/SignatureConstructedGenericType.cs b/src/System.Private.CoreLib/shared/System/Reflection/SignatureConstructedGenericType.cs
index d3d88520fc5..01408016bc2 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/SignatureConstructedGenericType.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/SignatureConstructedGenericType.cs
@@ -2,9 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using System;
+#nullable enable
 using System.Text;
-using System.Diagnostics;
 
 namespace System.Reflection
 {
@@ -14,16 +13,16 @@ internal sealed class SignatureConstructedGenericType : SignatureType
         // intended user of this constructor.
         internal SignatureConstructedGenericType(Type genericTypeDefinition, Type[] typeArguments)
         {
-            if (genericTypeDefinition == null)
+            if (genericTypeDefinition is null)
                 throw new ArgumentNullException(nameof(genericTypeDefinition));
 
-            if (typeArguments == null)
+            if (typeArguments is null)
                 throw new ArgumentNullException(nameof(typeArguments));
 
             typeArguments = (Type[])(typeArguments.Clone());
             for (int i = 0; i < typeArguments.Length; i++)
             {
-                if (typeArguments[i] == null)
+                if (typeArguments[i] is null)
                     throw new ArgumentNullException(nameof(typeArguments));
             }
 
@@ -57,15 +56,14 @@ public sealed override bool ContainsGenericParameters
             }
         }
     
-        internal sealed override SignatureType ElementType => null;
+        internal sealed override SignatureType? ElementType => null;
         public sealed override int GetArrayRank() => throw new ArgumentException(SR.Argument_HasToBeArrayClass);
         public sealed override Type GetGenericTypeDefinition() => _genericTypeDefinition;
         public sealed override Type[] GetGenericArguments() => GenericTypeArguments;
         public sealed override Type[] GenericTypeArguments => (Type[])(_genericTypeArguments.Clone());
         public sealed override int GenericParameterPosition => throw new InvalidOperationException(SR.Arg_NotGenericParameter);
-    
         public sealed override string Name => _genericTypeDefinition.Name;
-        public sealed override string Namespace => _genericTypeDefinition.Namespace;
+        public sealed override string? Namespace => _genericTypeDefinition.Namespace;
     
         public sealed override string ToString()
         {
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/SignatureGenericMethodParameterType.cs b/src/System.Private.CoreLib/shared/System/Reflection/SignatureGenericMethodParameterType.cs
index d0790283fb0..42ab16fe42d 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/SignatureGenericMethodParameterType.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/SignatureGenericMethodParameterType.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 namespace System.Reflection
 {
     internal sealed class SignatureGenericMethodParameterType : SignatureGenericParameterType
@@ -13,7 +14,7 @@ internal SignatureGenericMethodParameterType(int position)
 
         public sealed override bool IsGenericTypeParameter => false;
         public sealed override bool IsGenericMethodParameter => true;
-    
+
         public sealed override string Name => "!!" + GenericParameterPosition;
     }
 }
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/SignatureGenericParameterType.cs b/src/System.Private.CoreLib/shared/System/Reflection/SignatureGenericParameterType.cs
index fee7bce3530..15e5dfa7c25 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/SignatureGenericParameterType.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/SignatureGenericParameterType.cs
@@ -2,7 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using System;
+#nullable enable
 using System.Diagnostics;
 
 namespace System.Reflection
@@ -29,15 +29,14 @@ protected SignatureGenericParameterType(int position)
         public abstract override bool IsGenericMethodParameter { get; }
         public sealed override bool ContainsGenericParameters => true;
     
-        internal sealed override SignatureType ElementType => null;
+        internal sealed override SignatureType? ElementType => null;
         public sealed override int GetArrayRank() => throw new ArgumentException(SR.Argument_HasToBeArrayClass);
         public sealed override Type GetGenericTypeDefinition() => throw new InvalidOperationException(SR.InvalidOperation_NotGenericType);
         public sealed override Type[] GetGenericArguments() => Array.Empty<Type>();
         public sealed override Type[] GenericTypeArguments => Array.Empty<Type>();
         public sealed override int GenericParameterPosition => _position;
-    
         public abstract override string Name { get; }
-        public sealed override string Namespace => null;
+        public sealed override string? Namespace => null;
     
         public sealed override string ToString() => Name;
     
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/SignatureHasElementType.cs b/src/System.Private.CoreLib/shared/System/Reflection/SignatureHasElementType.cs
index e74e5f5aa28..f5c0a787852 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/SignatureHasElementType.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/SignatureHasElementType.cs
@@ -2,7 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using System;
+#nullable enable
 using System.Diagnostics;
 
 namespace System.Reflection
@@ -30,15 +30,14 @@ protected SignatureHasElementType(SignatureType elementType)
         public sealed override bool IsGenericMethodParameter => false;
         public sealed override bool ContainsGenericParameters => _elementType.ContainsGenericParameters;
     
-        internal sealed override SignatureType ElementType => _elementType;
+        internal sealed override SignatureType? ElementType => _elementType;
         public abstract override int GetArrayRank();
         public sealed override Type GetGenericTypeDefinition() => throw new InvalidOperationException(SR.InvalidOperation_NotGenericType);
         public sealed override Type[] GetGenericArguments() => Array.Empty<Type>();
         public sealed override Type[] GenericTypeArguments => Array.Empty<Type>();
         public sealed override int GenericParameterPosition => throw new InvalidOperationException(SR.Arg_NotGenericParameter);
-    
         public sealed override string Name => _elementType.Name + Suffix;
-        public sealed override string Namespace => _elementType.Namespace;
+        public sealed override string? Namespace => _elementType.Namespace;
     
         public sealed override string ToString() => _elementType.ToString() + Suffix;
     
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/SignaturePointerType.cs b/src/System.Private.CoreLib/shared/System/Reflection/SignaturePointerType.cs
index a75a2081658..d7130850b49 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/SignaturePointerType.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/SignaturePointerType.cs
@@ -2,8 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using System;
-
+#nullable enable
 namespace System.Reflection
 {
     internal sealed class SignaturePointerType : SignatureHasElementType
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/SignatureType.cs b/src/System.Private.CoreLib/shared/System/Reflection/SignatureType.cs
index 40a0590448b..c54e09c19d8 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/SignatureType.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/SignatureType.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Globalization;
 using System.Collections.Generic;
 using System.Runtime.InteropServices;
@@ -49,40 +50,48 @@ public sealed override Type MakeArrayType(int rank)
         public sealed override Type MakeGenericType(params Type[] typeArguments) => throw new NotSupportedException(SR.NotSupported_SignatureType); // There is no SignatureType for type definition types so it would never be legal to call this.
 
         // Dissectors
-        public sealed override Type GetElementType() => ElementType;
+        public sealed override Type? GetElementType() => ElementType;
         public abstract override int GetArrayRank();
         public abstract override Type GetGenericTypeDefinition();
         public abstract override Type[] GenericTypeArguments { get; }
         public abstract override Type[] GetGenericArguments();
         public abstract override int GenericParameterPosition { get; }
-        internal abstract SignatureType ElementType { get; }
+        internal abstract SignatureType? ElementType { get; }
 
         // Identity
 #if DEBUG
-        public sealed override bool Equals(object o) => base.Equals(o);
-        public sealed override bool Equals(Type o) => base.Equals(o);
+        public sealed override bool Equals(object? o) => base.Equals(o);
+        public sealed override bool Equals(Type? o) => base.Equals(o);
         public sealed override int GetHashCode() => base.GetHashCode();
 #endif
         public sealed override Type UnderlyingSystemType => this;  // Equals(Type) depends on this.
     
         // Naming and diagnostics
         public abstract override string Name { get; }
-        public abstract override string Namespace { get; }
-        public sealed override string FullName => null;
-        public sealed override string AssemblyQualifiedName => null;
+        public abstract override string? Namespace { get; }
+        public sealed override string? FullName => null;
+        public sealed override string? AssemblyQualifiedName => null;
         public abstract override string ToString();
 
         // Not supported on Signature Types
         public sealed override Assembly Assembly => throw new NotSupportedException(SR.NotSupported_SignatureType);
         public sealed override Module Module => throw new NotSupportedException(SR.NotSupported_SignatureType);
+
+#pragma warning disable CS8608 // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/23268
         public sealed override Type ReflectedType => throw new NotSupportedException(SR.NotSupported_SignatureType);
         public sealed override Type BaseType => throw new NotSupportedException(SR.NotSupported_SignatureType);
+#pragma warning restore CS8608
+
         public sealed override Type[] GetInterfaces() => throw new NotSupportedException(SR.NotSupported_SignatureType);
-        public sealed override bool IsAssignableFrom(Type c) => throw new NotSupportedException(SR.NotSupported_SignatureType);
+        public sealed override bool IsAssignableFrom(Type? c) => throw new NotSupportedException(SR.NotSupported_SignatureType);
         public sealed override int MetadataToken => throw new NotSupportedException(SR.NotSupported_SignatureType);
         public sealed override bool HasSameMetadataDefinitionAs(MemberInfo other) => throw new NotSupportedException(SR.NotSupported_SignatureType);
+
+#pragma warning disable CS8608 // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/23268
         public sealed override Type DeclaringType => throw new NotSupportedException(SR.NotSupported_SignatureType);
         public sealed override MethodBase DeclaringMethod => throw new NotSupportedException(SR.NotSupported_SignatureType);
+#pragma warning restore CS8608
+
         public sealed override Type[] GetGenericParameterConstraints() => throw new NotSupportedException(SR.NotSupported_SignatureType);
         public sealed override GenericParameterAttributes GenericParameterAttributes => throw new NotSupportedException(SR.NotSupported_SignatureType);
         public sealed override bool IsEnumDefined(object value) => throw new NotSupportedException(SR.NotSupported_SignatureType);
@@ -103,11 +112,11 @@ public sealed override Type MakeArrayType(int rank)
         public sealed override Type GetNestedType(string name, BindingFlags bindingAttr) => throw new NotSupportedException(SR.NotSupported_SignatureType);
         public sealed override Type[] GetNestedTypes(BindingFlags bindingAttr) => throw new NotSupportedException(SR.NotSupported_SignatureType);
         public sealed override PropertyInfo[] GetProperties(BindingFlags bindingAttr) => throw new NotSupportedException(SR.NotSupported_SignatureType);
-        public sealed override object InvokeMember(string name, BindingFlags invokeAttr, Binder binder, object target, object[] args, ParameterModifier[] modifiers, CultureInfo culture, string[] namedParameters) => throw new NotSupportedException(SR.NotSupported_SignatureType);
-        protected sealed override MethodInfo GetMethodImpl(string name, BindingFlags bindingAttr, Binder binder, CallingConventions callConvention, Type[] types, ParameterModifier[] modifiers) => throw new NotSupportedException(SR.NotSupported_SignatureType);
-        protected sealed override MethodInfo GetMethodImpl(string name, int genericParameterCount, BindingFlags bindingAttr, Binder binder, CallingConventions callConvention, Type[] types, ParameterModifier[] modifiers) => throw new NotSupportedException(SR.NotSupported_SignatureType);
-        protected sealed override PropertyInfo GetPropertyImpl(string name, BindingFlags bindingAttr, Binder binder, Type returnType, Type[] types, ParameterModifier[] modifiers) => throw new NotSupportedException(SR.NotSupported_SignatureType);
-        public sealed override MemberInfo[] FindMembers(MemberTypes memberType, BindingFlags bindingAttr, MemberFilter filter, object filterCriteria) => throw new NotSupportedException(SR.NotSupported_SignatureType);
+        public sealed override object InvokeMember(string name, BindingFlags invokeAttr, Binder? binder, object? target, object[]? args, ParameterModifier[]? modifiers, CultureInfo? culture, string[]? namedParameters) => throw new NotSupportedException(SR.NotSupported_SignatureType);
+        protected sealed override MethodInfo GetMethodImpl(string name, BindingFlags bindingAttr, Binder? binder, CallingConventions callConvention, Type[]? types, ParameterModifier[]? modifiers) => throw new NotSupportedException(SR.NotSupported_SignatureType);
+        protected sealed override MethodInfo GetMethodImpl(string name, int genericParameterCount, BindingFlags bindingAttr, Binder? binder, CallingConventions callConvention, Type[]? types, ParameterModifier[]? modifiers) => throw new NotSupportedException(SR.NotSupported_SignatureType);
+        protected sealed override PropertyInfo GetPropertyImpl(string name, BindingFlags bindingAttr, Binder? binder, Type? returnType, Type[]? types, ParameterModifier[]? modifiers) => throw new NotSupportedException(SR.NotSupported_SignatureType);
+        public sealed override MemberInfo[] FindMembers(MemberTypes memberType, BindingFlags bindingAttr, MemberFilter? filter, object filterCriteria) => throw new NotSupportedException(SR.NotSupported_SignatureType);
         public sealed override MemberInfo[] GetMember(string name, BindingFlags bindingAttr) => throw new NotSupportedException(SR.NotSupported_SignatureType);
         public sealed override MemberInfo[] GetMember(string name, MemberTypes type, BindingFlags bindingAttr) => throw new NotSupportedException(SR.NotSupported_SignatureType);
         public sealed override MemberInfo[] GetDefaultMembers() => throw new NotSupportedException(SR.NotSupported_SignatureType);
@@ -117,7 +126,7 @@ public sealed override Type MakeArrayType(int rank)
         public sealed override bool IsDefined(Type attributeType, bool inherit) => throw new NotSupportedException(SR.NotSupported_SignatureType);
         public sealed override IList<CustomAttributeData> GetCustomAttributesData() => throw new NotSupportedException(SR.NotSupported_SignatureType);
         public sealed override Type GetInterface(string name, bool ignoreCase) => throw new NotSupportedException(SR.NotSupported_SignatureType);
-        protected sealed override ConstructorInfo GetConstructorImpl(BindingFlags bindingAttr, Binder binder, CallingConventions callConvention, Type[] types, ParameterModifier[] modifiers) => throw new NotSupportedException(SR.NotSupported_SignatureType);
+        protected sealed override ConstructorInfo GetConstructorImpl(BindingFlags bindingAttr, Binder? binder, CallingConventions callConvention, Type[] types, ParameterModifier[]? modifiers) => throw new NotSupportedException(SR.NotSupported_SignatureType);
         protected sealed override bool IsCOMObjectImpl() => throw new NotSupportedException(SR.NotSupported_SignatureType);
         protected sealed override bool IsPrimitiveImpl() => throw new NotSupportedException(SR.NotSupported_SignatureType);
         public sealed override IEnumerable<CustomAttributeData> CustomAttributes => throw new NotSupportedException(SR.NotSupported_SignatureType);
@@ -125,8 +134,8 @@ public sealed override Type MakeArrayType(int rank)
         public sealed override InterfaceMapping GetInterfaceMap(Type interfaceType) => throw new NotSupportedException(SR.NotSupported_SignatureType);
         protected sealed override bool IsContextfulImpl() => throw new NotSupportedException(SR.NotSupported_SignatureType);
         public sealed override bool IsEnum => throw new NotSupportedException(SR.NotSupported_SignatureType);
-        public sealed override bool IsEquivalentTo(Type other) => throw new NotSupportedException(SR.NotSupported_SignatureType);
-        public sealed override bool IsInstanceOfType(object o) => throw new NotSupportedException(SR.NotSupported_SignatureType);
+        public sealed override bool IsEquivalentTo(Type? other) => throw new NotSupportedException(SR.NotSupported_SignatureType);
+        public sealed override bool IsInstanceOfType(object? o) => throw new NotSupportedException(SR.NotSupported_SignatureType);
         protected sealed override bool IsMarshalByRefImpl() => throw new NotSupportedException(SR.NotSupported_SignatureType);
         public sealed override bool IsSecurityCritical => throw new NotSupportedException(SR.NotSupported_SignatureType);
         public sealed override bool IsSecuritySafeCritical => throw new NotSupportedException(SR.NotSupported_SignatureType);
@@ -134,7 +143,11 @@ public sealed override Type MakeArrayType(int rank)
         public sealed override bool IsSerializable => throw new NotSupportedException(SR.NotSupported_SignatureType);
         public sealed override bool IsSubclassOf(Type c) => throw new NotSupportedException(SR.NotSupported_SignatureType);
         protected sealed override bool IsValueTypeImpl() => throw new NotSupportedException(SR.NotSupported_SignatureType);
+
+#pragma warning disable CS8608 // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/23268
         public sealed override StructLayoutAttribute StructLayoutAttribute => throw new NotSupportedException(SR.NotSupported_SignatureType);
+#pragma warning restore CS8608
+
         public sealed override RuntimeTypeHandle TypeHandle => throw new NotSupportedException(SR.NotSupported_SignatureType);
     }
 }
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/SignatureTypeExtensions.cs b/src/System.Private.CoreLib/shared/System/Reflection/SignatureTypeExtensions.cs
index 9247132546c..263dc50c7e1 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/SignatureTypeExtensions.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/SignatureTypeExtensions.cs
@@ -2,8 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using System;
-using System.Reflection;
+#nullable enable
 using System.Diagnostics;
 
 namespace System.Reflection
@@ -42,19 +41,19 @@ internal static bool MatchesExactly(this SignatureType pattern, Type actual)
         {
             if (pattern.IsSZArray)
             {
-                return actual.IsSZArray && pattern.ElementType.MatchesExactly(actual.GetElementType());
+                return actual.IsSZArray && pattern.ElementType!.MatchesExactly(actual.GetElementType()!);
             }
             else if (pattern.IsVariableBoundArray)
             {
-                return actual.IsVariableBoundArray && pattern.GetArrayRank() == actual.GetArrayRank() && pattern.ElementType.MatchesExactly(actual.GetElementType());
+                return actual.IsVariableBoundArray && pattern.GetArrayRank() == actual.GetArrayRank() && pattern.ElementType!.MatchesExactly(actual.GetElementType()!);
             }
             else if (pattern.IsByRef)
             {
-                return actual.IsByRef && pattern.ElementType.MatchesExactly(actual.GetElementType());
+                return actual.IsByRef && pattern.ElementType!.MatchesExactly(actual.GetElementType()!);
             }
             else if (pattern.IsPointer)
             {
-                return actual.IsPointer && pattern.ElementType.MatchesExactly(actual.GetElementType());
+                return actual.IsPointer && pattern.ElementType!.MatchesExactly(actual.GetElementType()!);
             }
             else if (pattern.IsConstructedGenericType)
             {
@@ -108,34 +107,34 @@ internal static bool MatchesExactly(this SignatureType pattern, Type actual)
         /// the method we're looking for, we return null rather than let the TypeLoadException bubble up. The DefaultBinder will catch
         /// the null and continue its search for a better candidate.
         /// </summary>
-        internal static Type TryResolveAgainstGenericMethod(this SignatureType signatureType, MethodInfo genericMethod)
+        internal static Type? TryResolveAgainstGenericMethod(this SignatureType signatureType, MethodInfo genericMethod)
         {
             return signatureType.TryResolve(genericMethod.GetGenericArguments());
         }
     
-        private static Type TryResolve(this SignatureType signatureType, Type[] genericMethodParameters)
+        private static Type? TryResolve(this SignatureType signatureType, Type[] genericMethodParameters)
         {
             if (signatureType.IsSZArray)
             {
-                return signatureType.ElementType.TryResolve(genericMethodParameters)?.TryMakeArrayType();
+                return signatureType.ElementType!.TryResolve(genericMethodParameters)?.TryMakeArrayType();
             }
             else if (signatureType.IsVariableBoundArray)
             {
-                return signatureType.ElementType.TryResolve(genericMethodParameters)?.TryMakeArrayType(signatureType.GetArrayRank());
+                return signatureType.ElementType!.TryResolve(genericMethodParameters)?.TryMakeArrayType(signatureType.GetArrayRank());
             }
             else if (signatureType.IsByRef)
             {
-                return signatureType.ElementType.TryResolve(genericMethodParameters)?.TryMakeByRefType();
+                return signatureType.ElementType!.TryResolve(genericMethodParameters)?.TryMakeByRefType();
             }
             else if (signatureType.IsPointer)
             {
-                return signatureType.ElementType.TryResolve(genericMethodParameters)?.TryMakePointerType();
+                return signatureType.ElementType!.TryResolve(genericMethodParameters)?.TryMakePointerType();
             }
             else if (signatureType.IsConstructedGenericType)
             {
                 Type[] genericTypeArguments = signatureType.GenericTypeArguments;
                 int count = genericTypeArguments.Length;
-                Type[] newGenericTypeArguments = new Type[count];
+                Type?[] newGenericTypeArguments = new Type[count];
                 for (int i = 0; i < count; i++)
                 {
                     Type genericTypeArgument = genericTypeArguments[i];
@@ -150,7 +149,7 @@ private static Type TryResolve(this SignatureType signatureType, Type[] genericM
                         newGenericTypeArguments[i] = genericTypeArgument;
                     }
                 }
-                return signatureType.GetGenericTypeDefinition().TryMakeGenericType(newGenericTypeArguments);
+                return signatureType.GetGenericTypeDefinition().TryMakeGenericType(newGenericTypeArguments!);
             }
             else if (signatureType.IsGenericMethodParameter)
             {
@@ -165,7 +164,7 @@ private static Type TryResolve(this SignatureType signatureType, Type[] genericM
             }
         }
     
-        private static Type TryMakeArrayType(this Type type)
+        private static Type? TryMakeArrayType(this Type type)
         {
             try
             {
@@ -177,7 +176,7 @@ private static Type TryMakeArrayType(this Type type)
             }
         }
     
-        private static Type TryMakeArrayType(this Type type, int rank)
+        private static Type? TryMakeArrayType(this Type type, int rank)
         {
             try
             {
@@ -189,7 +188,7 @@ private static Type TryMakeArrayType(this Type type, int rank)
             }
         }
     
-        private static Type TryMakeByRefType(this Type type)
+        private static Type? TryMakeByRefType(this Type type)
         {
             try
             {
@@ -201,7 +200,7 @@ private static Type TryMakeByRefType(this Type type)
             }
         }
     
-        private static Type TryMakePointerType(this Type type)
+        private static Type? TryMakePointerType(this Type type)
         {
             try
             {
@@ -213,7 +212,7 @@ private static Type TryMakePointerType(this Type type)
             }
         }
     
-        private static Type TryMakeGenericType(this Type type, Type[] instantiation)
+        private static Type? TryMakeGenericType(this Type type, Type[] instantiation)
         {
             try
             {
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/StrongNameKeyPair.cs b/src/System.Private.CoreLib/shared/System/Reflection/StrongNameKeyPair.cs
index a0ba97f835a..0f0abe57f6e 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/StrongNameKeyPair.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/StrongNameKeyPair.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.IO;
 using System.Runtime.Serialization;
 
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/TypeDelegator.cs b/src/System.Private.CoreLib/shared/System/Reflection/TypeDelegator.cs
index e0be6e87b65..f9df366879e 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/TypeDelegator.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/TypeDelegator.cs
@@ -6,26 +6,27 @@
 // 
 // This class wraps a Type object and delegates all methods to that Type.
 
+#nullable enable
 using CultureInfo = System.Globalization.CultureInfo;
 
 namespace System.Reflection
 {
     public class TypeDelegator : TypeInfo
     {
-        public override bool IsAssignableFrom(TypeInfo typeInfo)
+        public override bool IsAssignableFrom(TypeInfo? typeInfo)
         {
             if (typeInfo == null)
                 return false;
             return IsAssignableFrom(typeInfo.AsType());
         }
 
-        protected Type typeImpl;
+        protected Type typeImpl = null!;
 
         protected TypeDelegator() { }
 
         public TypeDelegator(Type delegatingType)
         {
-            if (delegatingType == null)
+            if (delegatingType is null)
                 throw new ArgumentNullException(nameof(delegatingType));
 
             typeImpl = delegatingType;
@@ -34,8 +35,8 @@ public TypeDelegator(Type delegatingType)
         public override Guid GUID => typeImpl.GUID;
         public override int MetadataToken => typeImpl.MetadataToken;
 
-        public override object InvokeMember(string name, BindingFlags invokeAttr, Binder binder, object target,
-            object[] args, ParameterModifier[] modifiers, CultureInfo culture, string[] namedParameters)
+        public override object? InvokeMember(string name, BindingFlags invokeAttr, Binder? binder, object? target,
+            object[]? args, ParameterModifier[]? modifiers, CultureInfo? culture, string[]? namedParameters)
         {
             return typeImpl.InvokeMember(name, invokeAttr, binder, target, args, modifiers, culture, namedParameters);
         }
@@ -44,21 +45,21 @@ public override object InvokeMember(string name, BindingFlags invokeAttr, Binder
         public override Assembly Assembly => typeImpl.Assembly;
         public override RuntimeTypeHandle TypeHandle => typeImpl.TypeHandle;
         public override string Name => typeImpl.Name;
-        public override string FullName => typeImpl.FullName;
-        public override string Namespace => typeImpl.Namespace;
-        public override string AssemblyQualifiedName => typeImpl.AssemblyQualifiedName;
-        public override Type BaseType => typeImpl.BaseType;
+        public override string? FullName => typeImpl.FullName;
+        public override string? Namespace => typeImpl.Namespace;
+        public override string? AssemblyQualifiedName => typeImpl.AssemblyQualifiedName;
+        public override Type? BaseType => typeImpl.BaseType;
 
-        protected override ConstructorInfo GetConstructorImpl(BindingFlags bindingAttr, Binder binder,
-                CallingConventions callConvention, Type[] types, ParameterModifier[] modifiers)
+        protected override ConstructorInfo? GetConstructorImpl(BindingFlags bindingAttr, Binder? binder,
+                CallingConventions callConvention, Type[] types, ParameterModifier[]? modifiers)
         {
             return typeImpl.GetConstructor(bindingAttr, binder, callConvention, types, modifiers);
         }
 
         public override ConstructorInfo[] GetConstructors(BindingFlags bindingAttr) => typeImpl.GetConstructors(bindingAttr);
 
-        protected override MethodInfo GetMethodImpl(string name, BindingFlags bindingAttr, Binder binder,
-                CallingConventions callConvention, Type[] types, ParameterModifier[] modifiers)
+        protected override MethodInfo? GetMethodImpl(string name, BindingFlags bindingAttr, Binder? binder,
+                CallingConventions callConvention, Type[]? types, ParameterModifier[]? modifiers)
         {
             // This is interesting there are two paths into the impl.  One that validates
             //  type as non-null and one where type may be null.
@@ -70,30 +71,30 @@ protected override MethodInfo GetMethodImpl(string name, BindingFlags bindingAtt
 
         public override MethodInfo[] GetMethods(BindingFlags bindingAttr) => typeImpl.GetMethods(bindingAttr);
 
-        public override FieldInfo GetField(string name, BindingFlags bindingAttr) => typeImpl.GetField(name, bindingAttr);
+        public override FieldInfo? GetField(string name, BindingFlags bindingAttr) => typeImpl.GetField(name, bindingAttr);
         public override FieldInfo[] GetFields(BindingFlags bindingAttr) => typeImpl.GetFields(bindingAttr);
 
-        public override Type GetInterface(string name, bool ignoreCase) => typeImpl.GetInterface(name, ignoreCase);
+        public override Type? GetInterface(string name, bool ignoreCase) => typeImpl.GetInterface(name, ignoreCase);
 
         public override Type[] GetInterfaces() => typeImpl.GetInterfaces();
 
-        public override EventInfo GetEvent(string name, BindingFlags bindingAttr) => typeImpl.GetEvent(name, bindingAttr);
+        public override EventInfo? GetEvent(string name, BindingFlags bindingAttr) => typeImpl.GetEvent(name, bindingAttr);
 
         public override EventInfo[] GetEvents() => typeImpl.GetEvents();
 
-        protected override PropertyInfo GetPropertyImpl(string name, BindingFlags bindingAttr, Binder binder,
-                        Type returnType, Type[] types, ParameterModifier[] modifiers)
+        protected override PropertyInfo? GetPropertyImpl(string name, BindingFlags bindingAttr, Binder? binder,
+                        Type? returnType, Type[]? types, ParameterModifier[]? modifiers)
         {
             if (returnType == null && types == null)
                 return typeImpl.GetProperty(name, bindingAttr);
             else
-                return typeImpl.GetProperty(name, bindingAttr, binder, returnType, types, modifiers);
+                return typeImpl.GetProperty(name, bindingAttr, binder, returnType, types!, modifiers);
         }
 
         public override PropertyInfo[] GetProperties(BindingFlags bindingAttr) => typeImpl.GetProperties(bindingAttr);
         public override EventInfo[] GetEvents(BindingFlags bindingAttr) => typeImpl.GetEvents(bindingAttr);
         public override Type[] GetNestedTypes(BindingFlags bindingAttr) => typeImpl.GetNestedTypes(bindingAttr);
-        public override Type GetNestedType(string name, BindingFlags bindingAttr) => typeImpl.GetNestedType(name, bindingAttr);
+        public override Type? GetNestedType(string name, BindingFlags bindingAttr) => typeImpl.GetNestedType(name, bindingAttr);
         public override MemberInfo[] GetMember(string name, MemberTypes type, BindingFlags bindingAttr) => typeImpl.GetMember(name, type, bindingAttr);
         public override MemberInfo[] GetMembers(BindingFlags bindingAttr) => typeImpl.GetMembers(bindingAttr);
 
@@ -115,7 +116,7 @@ protected override PropertyInfo GetPropertyImpl(string name, BindingFlags bindin
 
         public override bool IsCollectible => typeImpl.IsCollectible;
 
-        public override Type GetElementType() => typeImpl.GetElementType();
+        public override Type? GetElementType() => typeImpl.GetElementType();
         protected override bool HasElementTypeImpl() => typeImpl.HasElementType;
 
         public override Type UnderlyingSystemType => typeImpl.UnderlyingSystemType;
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/TypeFilter.cs b/src/System.Private.CoreLib/shared/System/Reflection/TypeFilter.cs
index eb049f81f90..ffb2f1fd1bd 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/TypeFilter.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/TypeFilter.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 namespace System.Reflection
 {
     public delegate bool TypeFilter(Type m, object filterCriteria);
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/TypeInfo.cs b/src/System.Private.CoreLib/shared/System/Reflection/TypeInfo.cs
index f4add736f46..99720732427 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/TypeInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/TypeInfo.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Collections.Generic;
 
 namespace System.Reflection
@@ -15,11 +16,11 @@ protected TypeInfo() { }
 
         public virtual Type[] GenericTypeParameters => IsGenericTypeDefinition ? GetGenericArguments() : Type.EmptyTypes;
 
-        public virtual EventInfo GetDeclaredEvent(string name) => GetEvent(name, TypeInfo.DeclaredOnlyLookup);
-        public virtual FieldInfo GetDeclaredField(string name) => GetField(name, TypeInfo.DeclaredOnlyLookup);
-        public virtual MethodInfo GetDeclaredMethod(string name) => GetMethod(name, TypeInfo.DeclaredOnlyLookup);
-        public virtual TypeInfo GetDeclaredNestedType(string name) => GetNestedType(name, TypeInfo.DeclaredOnlyLookup)?.GetTypeInfo();
-        public virtual PropertyInfo GetDeclaredProperty(string name) => GetProperty(name, TypeInfo.DeclaredOnlyLookup);
+        public virtual EventInfo? GetDeclaredEvent(string name) => GetEvent(name, TypeInfo.DeclaredOnlyLookup);
+        public virtual FieldInfo? GetDeclaredField(string name) => GetField(name, TypeInfo.DeclaredOnlyLookup);
+        public virtual MethodInfo? GetDeclaredMethod(string name) => GetMethod(name, TypeInfo.DeclaredOnlyLookup);
+        public virtual TypeInfo? GetDeclaredNestedType(string name) => GetNestedType(name, TypeInfo.DeclaredOnlyLookup)!.GetTypeInfo();
+        public virtual PropertyInfo? GetDeclaredProperty(string name) => GetProperty(name, TypeInfo.DeclaredOnlyLookup);
 
         public virtual IEnumerable<MethodInfo> GetDeclaredMethods(string name)
         {
@@ -50,7 +51,7 @@ public virtual IEnumerable<System.Reflection.TypeInfo> DeclaredNestedTypes
         public virtual IEnumerable<Type> ImplementedInterfaces => GetInterfaces();
 
         //a re-implementation of ISAF from Type, skipping the use of UnderlyingType
-        public virtual bool IsAssignableFrom(TypeInfo typeInfo)
+        public virtual bool IsAssignableFrom(TypeInfo? typeInfo)
         {
             if (typeInfo == null)
                 return false;
diff --git a/src/System.Private.CoreLib/shared/System/Resources/ManifestBasedResourceGroveler.cs b/src/System.Private.CoreLib/shared/System/Resources/ManifestBasedResourceGroveler.cs
index 569fcf93ef4..39d4272aec0 100644
--- a/src/System.Private.CoreLib/shared/System/Resources/ManifestBasedResourceGroveler.cs
+++ b/src/System.Private.CoreLib/shared/System/Resources/ManifestBasedResourceGroveler.cs
@@ -252,7 +252,7 @@ internal ResourceSet CreateResourceSet(Stream store, Assembly assembly)
                         }
                         else
                         {
-                            Type readerType = Type.GetType(readerTypeName, throwOnError: true);
+                            Type readerType = Type.GetType(readerTypeName, throwOnError: true)!;
                             object[] args = new object[1];
                             args[0] = store;
                             reader = (IResourceReader)Activator.CreateInstance(readerType, args)!;
@@ -265,7 +265,7 @@ internal ResourceSet CreateResourceSet(Stream store, Assembly assembly)
                         if (_mediator.UserResourceSet == null)
                         {
                             Debug.Assert(resSetTypeName != null, "We should have a ResourceSet type name from the custom resource file here.");
-                            resSetType = Type.GetType(resSetTypeName, true, false);
+                            resSetType = Type.GetType(resSetTypeName, true, false)!;
                         }
                         else
                         {
@@ -325,7 +325,7 @@ internal ResourceSet CreateResourceSet(Stream store, Assembly assembly)
             Debug.Assert(satellite != null, "satellite shouldn't be null; check caller");
             Debug.Assert(fileName != null, "fileName shouldn't be null; check caller");
 
-            Stream? stream = satellite.GetManifestResourceStream(_mediator.LocationInfo, fileName);
+            Stream? stream = satellite.GetManifestResourceStream(_mediator.LocationInfo!, fileName);
             if (stream == null)
             {
                 stream = CaseInsensitiveManifestResourceStreamLookup(satellite, fileName);
diff --git a/src/System.Private.CoreLib/shared/System/Resources/ResourceManager.Uap.cs b/src/System.Private.CoreLib/shared/System/Resources/ResourceManager.Uap.cs
index 7437ac96676..6488855b72c 100644
--- a/src/System.Private.CoreLib/shared/System/Resources/ResourceManager.Uap.cs
+++ b/src/System.Private.CoreLib/shared/System/Resources/ResourceManager.Uap.cs
@@ -71,7 +71,7 @@ public partial class ResourceManager
         internal static WindowsRuntimeResourceManagerBase GetWinRTResourceManager()
         {
 #if FEATURE_APPX
-            Type WinRTResourceManagerType = Type.GetType("System.Resources.WindowsRuntimeResourceManager, System.Runtime.WindowsRuntime", throwOnError: true);
+            Type WinRTResourceManagerType = Type.GetType("System.Resources.WindowsRuntimeResourceManager, System.Runtime.WindowsRuntime", throwOnError: true)!;
 #else // ENABLE_WINRT
             Assembly hiddenScopeAssembly = Assembly.Load(Internal.Runtime.Augments.RuntimeAugments.HiddenScopeAssemblyName);
             Type WinRTResourceManagerType = hiddenScopeAssembly.GetType("System.Resources.WindowsRuntimeResourceManager", true);
diff --git a/src/System.Private.CoreLib/shared/System/Resources/ResourceManager.cs b/src/System.Private.CoreLib/shared/System/Resources/ResourceManager.cs
index ec5877a6c7b..713f4e600e6 100644
--- a/src/System.Private.CoreLib/shared/System/Resources/ResourceManager.cs
+++ b/src/System.Private.CoreLib/shared/System/Resources/ResourceManager.cs
@@ -417,7 +417,7 @@ protected virtual string GetResourceFileName(CultureInfo culture)
             {
                 string fileName = GetResourceFileName(culture);
                 Debug.Assert(MainAssembly != null);
-                Stream stream = MainAssembly.GetManifestResourceStream(_locationInfo, fileName);
+                Stream? stream = MainAssembly.GetManifestResourceStream(_locationInfo!, fileName);
                 if (createIfNotExists && stream != null)
                 {
                     rs = ((ManifestBasedResourceGroveler)_resourceGroveler).CreateResourceSet(stream, MainAssembly);
diff --git a/src/System.Private.CoreLib/shared/System/Resources/ResourceReader.cs b/src/System.Private.CoreLib/shared/System/Resources/ResourceReader.cs
index 156007b14f2..4b280924ea6 100644
--- a/src/System.Private.CoreLib/shared/System/Resources/ResourceReader.cs
+++ b/src/System.Private.CoreLib/shared/System/Resources/ResourceReader.cs
@@ -21,10 +21,10 @@ namespace System.Resources
     // Provides the default implementation of IResourceReader, reading
     // .resources file from the system default binary format.  This class
     // can be treated as an enumerator once.
-    // 
-    // See the RuntimeResourceSet overview for details on the system 
+    //
+    // See the RuntimeResourceSet overview for details on the system
     // default file format.
-    // 
+    //
 
     internal struct ResourceLocator
     {
@@ -73,14 +73,14 @@ public sealed partial class
 
         private BinaryReader _store;    // backing store we're reading from.
         // Used by RuntimeResourceSet and this class's enumerator.  Maps
-        // resource name to a value, a ResourceLocator, or a 
+        // resource name to a value, a ResourceLocator, or a
         // LooselyLinkedManifestResource.
         internal Dictionary<string, ResourceLocator>? _resCache;
         private long _nameSectionOffset;  // Offset to name section of file.
         private long _dataSectionOffset;  // Offset to Data section of file.
 
         // Note this class is tightly coupled with UnmanagedMemoryStream.
-        // At runtime when getting an embedded resource from an assembly, 
+        // At runtime when getting an embedded resource from an assembly,
         // we're given an UnmanagedMemoryStream referring to the mmap'ed portion
         // of the assembly.  The pointers here are pointers into that block of
         // memory controlled by the OS's loader.
@@ -158,7 +158,7 @@ private unsafe void Dispose(bool disposing)
                 _resCache = null;
                 if (disposing)
                 {
-                    // Close the stream in a thread-safe way.  This fix means 
+                    // Close the stream in a thread-safe way.  This fix means
                     // that we may call Close n times, but that's safe.
                     BinaryReader copyOfStore = _store;
                     _store = null!; // TODO-NULLABLE: dispose should not null this out
@@ -256,7 +256,7 @@ internal int FindPosForResource(string name)
             Debug.Assert(_store != null, "ResourceReader is closed!");
             int hash = FastResourceComparer.HashFunction(name);
 
-            // Binary search over the hashes.  Use the _namePositions array to 
+            // Binary search over the hashes.  Use the _namePositions array to
             // determine where they exist in the underlying stream.
             int lo = 0;
             int hi = _numResources - 1;
@@ -266,7 +266,7 @@ internal int FindPosForResource(string name)
             {
                 index = (lo + hi) >> 1;
                 // Do NOT use subtraction here, since it will wrap for large
-                // negative numbers. 
+                // negative numbers.
                 int currentHash = GetNameHash(index);
                 int c;
                 if (currentHash == hash)
@@ -291,9 +291,9 @@ internal int FindPosForResource(string name)
                 return -1;
             }
 
-            // index is the location in our hash array that corresponds with a 
+            // index is the location in our hash array that corresponds with a
             // value in the namePositions array.
-            // There could be collisions in our hash function.  Check on both sides 
+            // There could be collisions in our hash function.  Check on both sides
             // of index to find the range of hash values that are equal to the
             // target hash value.
             if (lo != index)
@@ -329,7 +329,7 @@ internal int FindPosForResource(string name)
         }
 
         // This compares the String in the .resources file at the current position
-        // with the string you pass in. 
+        // with the string you pass in.
         // Whoever calls this method should make sure that they take a lock
         // so no one else can cause us to seek in the stream.
         private unsafe bool CompareStringEqualsName(string name)
@@ -457,7 +457,7 @@ private unsafe string AllocateStringForNameIndex(int index, out int dataOffset)
 
         // This takes a virtual offset into the data section and reads a String
         // from that location.
-        // Anyone who calls LoadObject should make sure they take a lock so 
+        // Anyone who calls LoadObject should make sure they take a lock so
         // no one can cause us to do a seek in here.
         internal string? LoadString(int pos)
         {
@@ -478,7 +478,7 @@ private unsafe string AllocateStringForNameIndex(int index, out int dataOffset)
                 ResourceTypeCode typeCode = (ResourceTypeCode)typeIndex;
                 if (typeCode != ResourceTypeCode.String && typeCode != ResourceTypeCode.Null)
                 {
-                    string typeString;
+                    string? typeString;
                     if (typeCode < ResourceTypeCode.StartOfUserTypes)
                         typeString = typeCode.ToString();
                     else
@@ -513,7 +513,7 @@ private unsafe string AllocateStringForNameIndex(int index, out int dataOffset)
 
         // This takes a virtual offset into the data section and reads an Object
         // from that location.
-        // Anyone who calls LoadObject should make sure they take a lock so 
+        // Anyone who calls LoadObject should make sure they take a lock so
         // no one can cause us to do a seek in here.
         internal object? LoadObjectV1(int pos)
         {
@@ -522,7 +522,7 @@ private unsafe string AllocateStringForNameIndex(int index, out int dataOffset)
 
             try
             {
-                // mega try-catch performs exceptionally bad on x64; factored out body into 
+                // mega try-catch performs exceptionally bad on x64; factored out body into
                 // _LoadObjectV1 and wrap here.
                 return _LoadObjectV1(pos);
             }
@@ -543,8 +543,8 @@ private unsafe string AllocateStringForNameIndex(int index, out int dataOffset)
             if (typeIndex == -1)
                 return null;
             Type type = FindType(typeIndex);
-            // Consider putting in logic to see if this type is a 
-            // primitive or a value type first, so we can reach the 
+            // Consider putting in logic to see if this type is a
+            // primitive or a value type first, so we can reach the
             // deserialization code faster for arbitrary objects.
 
             if (type == typeof(string))
@@ -597,7 +597,7 @@ private unsafe string AllocateStringForNameIndex(int index, out int dataOffset)
 
             try
             {
-                // mega try-catch performs exceptionally bad on x64; factored out body into 
+                // mega try-catch performs exceptionally bad on x64; factored out body into
                 // _LoadObjectV2 and wrap here.
                 return _LoadObjectV2(pos, out typeCode);
             }
@@ -715,7 +715,7 @@ private unsafe string AllocateStringForNameIndex(int index, out int dataOffset)
                             return new PinnedBufferMemoryStream(bytes);
                         }
 
-                        // make sure we don't create an UnmanagedMemoryStream that is longer than the resource stream. 
+                        // make sure we don't create an UnmanagedMemoryStream that is longer than the resource stream.
                         if (len > _ums.Length - _ums.Position)
                         {
                             throw new BadImageFormatException(SR.Format(SR.BadImageFormat_ResourceDataLengthInvalid, len));
@@ -751,7 +751,7 @@ private void ReadResources()
 
             try
             {
-                // mega try-catch performs exceptionally bad on x64; factored out body into 
+                // mega try-catch performs exceptionally bad on x64; factored out body into
                 // _ReadResources and wrap here.
                 _ReadResources();
             }
@@ -832,8 +832,8 @@ private void _ReadResources()
             }
 
             // Prepare to read in the array of name hashes
-            //  Note that the name hashes array is aligned to 8 bytes so 
-            //  we can use pointers into it on 64 bit machines. (4 bytes 
+            //  Note that the name hashes array is aligned to 8 bytes so
+            //  we can use pointers into it on 64 bit machines. (4 bytes
             //  may be sufficient, but let's plan for the future)
             //  Skip over alignment stuff.  All public .resources files
             //  should be aligned   No need to verify the byte values.
@@ -922,7 +922,7 @@ private void _ReadResources()
             }
         }
 
-        // This allows us to delay-initialize the Type[].  This might be a 
+        // This allows us to delay-initialize the Type[].  This might be a
         // good startup time savings, since we might have to load assemblies
         // and initialize Reflection.
         private Type FindType(int typeIndex)
@@ -940,15 +940,15 @@ private Type FindType(int typeIndex)
                     string typeName = _store.ReadString();
                     _typeTable[typeIndex] = Type.GetType(typeName, true);
                 }
-                // If serialization isn't supported, we convert FileNotFoundException to 
-                // NotSupportedException for consistency with v2. This is a corner-case, but the 
+                // If serialization isn't supported, we convert FileNotFoundException to
+                // NotSupportedException for consistency with v2. This is a corner-case, but the
                 // idea is that we want to give the user a more accurate error message. Even if
                 // the dependency were found, we know it will require serialization since it
                 // can't be one of the types we special case. So if the dependency were found,
-                // it would go down the serialization code path, resulting in NotSupported for 
+                // it would go down the serialization code path, resulting in NotSupported for
                 // SKUs without serialization.
                 //
-                // We don't want to regress the expected case by checking the type info before 
+                // We don't want to regress the expected case by checking the type info before
                 // getting to Type.GetType -- this is costly with v1 resource formats.
                 catch (FileNotFoundException)
                 {
diff --git a/src/System.Private.CoreLib/shared/System/Runtime/CompilerServices/RuntimeHelpers.cs b/src/System.Private.CoreLib/shared/System/Runtime/CompilerServices/RuntimeHelpers.cs
index 2ec8b760bdb..f3fb361021e 100644
--- a/src/System.Private.CoreLib/shared/System/Runtime/CompilerServices/RuntimeHelpers.cs
+++ b/src/System.Private.CoreLib/shared/System/Runtime/CompilerServices/RuntimeHelpers.cs
@@ -45,7 +45,7 @@ ref Unsafe.Add(ref Unsafe.As<byte, T>(ref array.GetRawSzArrayData()), offset),
             else
             {
                 // The array is actually a U[] where U:T.
-                T[] dest = (T[])Array.CreateInstance(array.GetType().GetElementType(), length);
+                T[] dest = (T[])Array.CreateInstance(array.GetType().GetElementType()!, length);
                 Array.Copy(array, offset, dest, 0, length);
                 return dest;
             }
diff --git a/src/System.Private.CoreLib/shared/System/Runtime/InteropServices/ComSourceInterfacesAttribute.cs b/src/System.Private.CoreLib/shared/System/Runtime/InteropServices/ComSourceInterfacesAttribute.cs
index b5ea55c2813..aa1a6a283ce 100644
--- a/src/System.Private.CoreLib/shared/System/Runtime/InteropServices/ComSourceInterfacesAttribute.cs
+++ b/src/System.Private.CoreLib/shared/System/Runtime/InteropServices/ComSourceInterfacesAttribute.cs
@@ -15,7 +15,7 @@ public ComSourceInterfacesAttribute(string sourceInterfaces)
 
         public ComSourceInterfacesAttribute(Type sourceInterface)
         {
-            Value = sourceInterface.FullName;
+            Value = sourceInterface.FullName!;
         }
 
         public ComSourceInterfacesAttribute(Type sourceInterface1, Type sourceInterface2)
diff --git a/src/System.Private.CoreLib/shared/System/Runtime/InteropServices/Marshal.cs b/src/System.Private.CoreLib/shared/System/Runtime/InteropServices/Marshal.cs
index 7cdab7b2aa7..afe298e0b3b 100644
--- a/src/System.Private.CoreLib/shared/System/Runtime/InteropServices/Marshal.cs
+++ b/src/System.Private.CoreLib/shared/System/Runtime/InteropServices/Marshal.cs
@@ -831,7 +831,7 @@ public static Guid GenerateGuidForType(Type type)
         /// a PROGID in the metadata then it is returned otherwise a stable PROGID
         /// is generated based on the fully qualified name of the type.
         /// </summary>
-        public static string GenerateProgIdForType(Type type)
+        public static string? GenerateProgIdForType(Type type)
         {
             if (type is null)
             {
diff --git a/src/System.Private.CoreLib/shared/System/Runtime/Serialization/SerializationInfo.cs b/src/System.Private.CoreLib/shared/System/Runtime/Serialization/SerializationInfo.cs
index e30e7d394ac..e3e7c930d89 100644
--- a/src/System.Private.CoreLib/shared/System/Runtime/Serialization/SerializationInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Runtime/Serialization/SerializationInfo.cs
@@ -176,8 +176,8 @@ public SerializationInfo(Type type, IFormatterConverter converter)
             }
 
             _rootType = type;
-            _rootTypeName = type.FullName;
-            _rootTypeAssemblyName = type.Module.Assembly.FullName;
+            _rootTypeName = type.FullName!;
+            _rootTypeAssemblyName = type.Module.Assembly.FullName!;
 
             _names = new string[DefaultSize];
             _values = new object[DefaultSize];
@@ -238,8 +238,8 @@ public void SetType(Type type)
             if (!ReferenceEquals(_rootType, type))
             {
                 _rootType = type;
-                _rootTypeName = type.FullName;
-                _rootTypeAssemblyName = type.Module.Assembly.FullName;
+                _rootTypeName = type.FullName!;
+                _rootTypeAssemblyName = type.Module.Assembly.FullName!;
                 IsFullTypeNameSetExplicit = false;
                 IsAssemblyNameSetExplicit = false;
             }
diff --git a/src/System.Private.CoreLib/shared/System/Type.Enum.cs b/src/System.Private.CoreLib/shared/System/Type.Enum.cs
index 84a64827093..006f8e94cff 100644
--- a/src/System.Private.CoreLib/shared/System/Type.Enum.cs
+++ b/src/System.Private.CoreLib/shared/System/Type.Enum.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Reflection;
 using System.Collections;
 using System.Collections.Generic;
@@ -64,7 +65,7 @@ public virtual bool IsEnumDefined(object value)
             }
         }
 
-        public virtual string GetEnumName(object value)
+        public virtual string? GetEnumName(object value)
         {
             if (value == null)
                 throw new ArgumentNullException(nameof(value));
@@ -162,7 +163,7 @@ private static int BinarySearch(Array array, object value)
         {
             ulong[] ulArray = new ulong[array.Length];
             for (int i = 0; i < array.Length; ++i)
-                ulArray[i] = Enum.ToUInt64(array.GetValue(i));
+                ulArray[i] = Enum.ToUInt64(array.GetValue(i)!);
 
             ulong ulValue = Enum.ToUInt64(value);
 
diff --git a/src/System.Private.CoreLib/shared/System/Type.Helpers.cs b/src/System.Private.CoreLib/shared/System/Type.Helpers.cs
index 1d3c3b8e8e3..c83b8f84498 100644
--- a/src/System.Private.CoreLib/shared/System/Type.Helpers.cs
+++ b/src/System.Private.CoreLib/shared/System/Type.Helpers.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Reflection;
 
 namespace System
@@ -16,7 +17,7 @@ public virtual bool IsSerializable
                 if ((GetAttributeFlagsImpl() & TypeAttributes.Serializable) != 0)
                     return true;
 
-                Type underlyingType = UnderlyingSystemType;
+                Type? underlyingType = UnderlyingSystemType;
                 if (underlyingType.IsRuntimeImplemented())
                 {
                     do
@@ -66,7 +67,7 @@ internal Type GetRootElementType()
             Type rootElementType = this;
 
             while (rootElementType.HasElementType)
-                rootElementType = rootElementType.GetElementType();
+                rootElementType = rootElementType.GetElementType()!;
 
             return rootElementType;
         }
@@ -76,8 +77,7 @@ public bool IsVisible
             get
             {
 #if CORECLR
-                RuntimeType rt = this as RuntimeType;
-                if (rt != null)
+                if (this is RuntimeType rt)
                     return RuntimeTypeHandle.IsVisible(rt);
 #endif //CORECLR
 
@@ -85,7 +85,7 @@ public bool IsVisible
                     return true;
 
                 if (HasElementType)
-                    return GetElementType().IsVisible;
+                    return GetElementType()!.IsVisible;
 
                 Type type = this;
                 while (type.IsNested)
@@ -94,7 +94,7 @@ public bool IsVisible
                         return false;
 
                     // this should be null for non-nested types.
-                    type = type.DeclaringType;
+                    type = type.DeclaringType!;
                 }
 
                 // Now "type" should be a top level type
@@ -119,37 +119,37 @@ public virtual Type[] FindInterfaces(TypeFilter filter, object filterCriteria)
             if (filter == null)
                 throw new ArgumentNullException(nameof(filter));
 
-            Type[] c = GetInterfaces();
+            Type?[] c = GetInterfaces();
             int cnt = 0;
             for (int i = 0; i < c.Length; i++)
             {
-                if (!filter(c[i], filterCriteria))
+                if (!filter(c[i]!, filterCriteria))
                     c[i] = null;
                 else
                     cnt++;
             }
             if (cnt == c.Length)
-                return c;
+                return c!;
 
             Type[] ret = new Type[cnt];
             cnt = 0;
             for (int i = 0; i < c.Length; i++)
             {
                 if (c[i] != null)
-                    ret[cnt++] = c[i];
+                    ret[cnt++] = c[i]!; // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/34644
             }
             return ret;
         }
 
-        public virtual MemberInfo[] FindMembers(MemberTypes memberType, BindingFlags bindingAttr, MemberFilter filter, object filterCriteria)
+        public virtual MemberInfo[] FindMembers(MemberTypes memberType, BindingFlags bindingAttr, MemberFilter? filter, object filterCriteria)
         {
             // Define the work arrays
-            MethodInfo[] m = null;
-            ConstructorInfo[] c = null;
-            FieldInfo[] f = null;
-            PropertyInfo[] p = null;
-            EventInfo[] e = null;
-            Type[] t = null;
+            MethodInfo?[]? m = null;
+            ConstructorInfo?[]? c = null;
+            FieldInfo?[]? f = null;
+            PropertyInfo?[]? p = null;
+            EventInfo?[]? e = null;
+            Type?[]? t = null;
 
             int i = 0;
             int cnt = 0;            // Total Matchs
@@ -161,7 +161,7 @@ public virtual MemberInfo[] FindMembers(MemberTypes memberType, BindingFlags bin
                 if (filter != null)
                 {
                     for (i = 0; i < m.Length; i++)
-                        if (!filter(m[i], filterCriteria))
+                        if (!filter(m[i]!, filterCriteria))
                             m[i] = null;
                         else
                             cnt++;
@@ -179,7 +179,7 @@ public virtual MemberInfo[] FindMembers(MemberTypes memberType, BindingFlags bin
                 if (filter != null)
                 {
                     for (i = 0; i < c.Length; i++)
-                        if (!filter(c[i], filterCriteria))
+                        if (!filter(c[i]!, filterCriteria))
                             c[i] = null;
                         else
                             cnt++;
@@ -197,7 +197,7 @@ public virtual MemberInfo[] FindMembers(MemberTypes memberType, BindingFlags bin
                 if (filter != null)
                 {
                     for (i = 0; i < f.Length; i++)
-                        if (!filter(f[i], filterCriteria))
+                        if (!filter(f[i]!, filterCriteria))
                             f[i] = null;
                         else
                             cnt++;
@@ -215,7 +215,7 @@ public virtual MemberInfo[] FindMembers(MemberTypes memberType, BindingFlags bin
                 if (filter != null)
                 {
                     for (i = 0; i < p.Length; i++)
-                        if (!filter(p[i], filterCriteria))
+                        if (!filter(p[i]!, filterCriteria))
                             p[i] = null;
                         else
                             cnt++;
@@ -233,7 +233,7 @@ public virtual MemberInfo[] FindMembers(MemberTypes memberType, BindingFlags bin
                 if (filter != null)
                 {
                     for (i = 0; i < e.Length; i++)
-                        if (!filter(e[i], filterCriteria))
+                        if (!filter(e[i]!, filterCriteria))
                             e[i] = null;
                         else
                             cnt++;
@@ -251,7 +251,7 @@ public virtual MemberInfo[] FindMembers(MemberTypes memberType, BindingFlags bin
                 if (filter != null)
                 {
                     for (i = 0; i < t.Length; i++)
-                        if (!filter(t[i], filterCriteria))
+                        if (!filter(t[i]!, filterCriteria))
                             t[i] = null;
                         else
                             cnt++;
@@ -271,7 +271,7 @@ public virtual MemberInfo[] FindMembers(MemberTypes memberType, BindingFlags bin
             {
                 for (i = 0; i < m.Length; i++)
                     if (m[i] != null)
-                        ret[cnt++] = m[i];
+                        ret[cnt++] = m[i]!;  // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/34644
             }
 
             // Copy the Constructors
@@ -279,7 +279,7 @@ public virtual MemberInfo[] FindMembers(MemberTypes memberType, BindingFlags bin
             {
                 for (i = 0; i < c.Length; i++)
                     if (c[i] != null)
-                        ret[cnt++] = c[i];
+                        ret[cnt++] = c[i]!;  // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/34644
             }
 
             // Copy the Fields
@@ -287,7 +287,7 @@ public virtual MemberInfo[] FindMembers(MemberTypes memberType, BindingFlags bin
             {
                 for (i = 0; i < f.Length; i++)
                     if (f[i] != null)
-                        ret[cnt++] = f[i];
+                        ret[cnt++] = f[i]!;  // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/34644
             }
 
             // Copy the Properties
@@ -295,7 +295,7 @@ public virtual MemberInfo[] FindMembers(MemberTypes memberType, BindingFlags bin
             {
                 for (i = 0; i < p.Length; i++)
                     if (p[i] != null)
-                        ret[cnt++] = p[i];
+                        ret[cnt++] = p[i]!; // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/34644
             }
 
             // Copy the Events
@@ -303,7 +303,7 @@ public virtual MemberInfo[] FindMembers(MemberTypes memberType, BindingFlags bin
             {
                 for (i = 0; i < e.Length; i++)
                     if (e[i] != null)
-                        ret[cnt++] = e[i];
+                        ret[cnt++] = e[i]!;  // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/34644
             }
 
             // Copy the Types
@@ -311,7 +311,7 @@ public virtual MemberInfo[] FindMembers(MemberTypes memberType, BindingFlags bin
             {
                 for (i = 0; i < t.Length; i++)
                     if (t[i] != null)
-                        ret[cnt++] = t[i];
+                        ret[cnt++] = t[i]!;  // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/34644
             }
 
             return ret;
@@ -319,7 +319,7 @@ public virtual MemberInfo[] FindMembers(MemberTypes memberType, BindingFlags bin
 
         public virtual bool IsSubclassOf(Type c)
         {
-            Type p = this;
+            Type? p = this;
             if (p == c)
                 return false;
             while (p != null)
@@ -331,7 +331,7 @@ public virtual bool IsSubclassOf(Type c)
             return false;
         }
 
-        public virtual bool IsAssignableFrom(Type c)
+        public virtual bool IsAssignableFrom(Type? c)
         {
             if (c == null)
                 return false;
@@ -368,7 +368,7 @@ public virtual bool IsAssignableFrom(Type c)
 
         internal bool ImplementInterface(Type ifaceType)
         {
-            Type t = this;
+            Type? t = this;
             while (t != null)
             {
                 Type[] interfaces = t.GetInterfaces();
diff --git a/src/System.Private.CoreLib/shared/System/Type.cs b/src/System.Private.CoreLib/shared/System/Type.cs
index d2a31cbf675..6162e12383f 100644
--- a/src/System.Private.CoreLib/shared/System/Type.cs
+++ b/src/System.Private.CoreLib/shared/System/Type.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
 using System.Threading;
 using System.Reflection;
 using System.Diagnostics;
@@ -18,18 +19,18 @@ protected Type() { }
 
         public new Type GetType() => base.GetType();
 
-        public abstract string Namespace { get; }
-        public abstract string AssemblyQualifiedName { get; }
-        public abstract string FullName { get; }
+        public abstract string? Namespace { get; }
+        public abstract string? AssemblyQualifiedName { get; }
+        public abstract string? FullName { get; }
 
         public abstract Assembly Assembly { get; }
         public abstract new Module Module { get; }
 
         public bool IsNested => DeclaringType != null;
-        public override Type DeclaringType => null;
-        public virtual MethodBase DeclaringMethod => null;
+        public override Type? DeclaringType => null;
+        public virtual MethodBase? DeclaringMethod => null;
 
-        public override Type ReflectedType => null;
+        public override Type? ReflectedType => null;
         public abstract Type UnderlyingSystemType { get; }
 
         public virtual bool IsTypeDefinition { get { throw NotImplemented.ByDesign; } }
@@ -41,7 +42,7 @@ protected Type() { }
         protected abstract bool IsPointerImpl();
         public virtual bool IsConstructedGenericType { get { throw NotImplemented.ByDesign; } }
         public virtual bool IsGenericParameter => false;
-        public virtual bool IsGenericTypeParameter => IsGenericParameter && DeclaringMethod == null;
+        public virtual bool IsGenericTypeParameter => IsGenericParameter && DeclaringMethod is null;
         public virtual bool IsGenericMethodParameter => IsGenericParameter && DeclaringMethod != null;
         public virtual bool IsGenericType => false;
         public virtual bool IsGenericTypeDefinition => false;
@@ -53,7 +54,7 @@ protected Type() { }
 
         public bool HasElementType => HasElementTypeImpl();
         protected abstract bool HasElementTypeImpl();
-        public abstract Type GetElementType();
+        public abstract Type? GetElementType();
 
         public virtual int GetArrayRank() { throw new NotSupportedException(SR.NotSupported_SubclassOverride); }
 
@@ -116,12 +117,12 @@ public virtual Type[] GetGenericParameterConstraints()
         public virtual bool IsSecuritySafeCritical { get { throw NotImplemented.ByDesign; } }
         public virtual bool IsSecurityTransparent { get { throw NotImplemented.ByDesign; } }
 
-        public virtual StructLayoutAttribute StructLayoutAttribute { get { throw new NotSupportedException(); } }
-        public ConstructorInfo TypeInitializer => GetConstructorImpl(BindingFlags.Static | BindingFlags.Public | BindingFlags.NonPublic, null, CallingConventions.Any, Type.EmptyTypes, null);
+        public virtual StructLayoutAttribute? StructLayoutAttribute { get { throw new NotSupportedException(); } }
+        public ConstructorInfo? TypeInitializer => GetConstructorImpl(BindingFlags.Static | BindingFlags.Public | BindingFlags.NonPublic, null, CallingConventions.Any, Type.EmptyTypes, null);
 
-        public ConstructorInfo GetConstructor(Type[] types) => GetConstructor(BindingFlags.Public | BindingFlags.Instance, null, types, null);
-        public ConstructorInfo GetConstructor(BindingFlags bindingAttr, Binder binder, Type[] types, ParameterModifier[] modifiers) => GetConstructor(bindingAttr, binder, CallingConventions.Any, types, modifiers);
-        public ConstructorInfo GetConstructor(BindingFlags bindingAttr, Binder binder, CallingConventions callConvention, Type[] types, ParameterModifier[] modifiers)
+        public ConstructorInfo? GetConstructor(Type[] types) => GetConstructor(BindingFlags.Public | BindingFlags.Instance, null, types, null);
+        public ConstructorInfo? GetConstructor(BindingFlags bindingAttr, Binder? binder, Type[] types, ParameterModifier[]? modifiers) => GetConstructor(bindingAttr, binder, CallingConventions.Any, types, modifiers);
+        public ConstructorInfo? GetConstructor(BindingFlags bindingAttr, Binder? binder, CallingConventions callConvention, Type[] types, ParameterModifier[]? modifiers)
         {
             if (types == null)
                 throw new ArgumentNullException(nameof(types));
@@ -132,19 +133,19 @@ public ConstructorInfo GetConstructor(BindingFlags bindingAttr, Binder binder, C
             }
             return GetConstructorImpl(bindingAttr, binder, callConvention, types, modifiers);
         }
-        protected abstract ConstructorInfo GetConstructorImpl(BindingFlags bindingAttr, Binder binder, CallingConventions callConvention, Type[] types, ParameterModifier[] modifiers);
+        protected abstract ConstructorInfo? GetConstructorImpl(BindingFlags bindingAttr, Binder? binder, CallingConventions callConvention, Type[] types, ParameterModifier[]? modifiers);
 
         public ConstructorInfo[] GetConstructors() => GetConstructors(BindingFlags.Public | BindingFlags.Instance);
         public abstract ConstructorInfo[] GetConstructors(BindingFlags bindingAttr);
 
-        public EventInfo GetEvent(string name) => GetEvent(name, Type.DefaultLookup);
-        public abstract EventInfo GetEvent(string name, BindingFlags bindingAttr);
+        public EventInfo? GetEvent(string name) => GetEvent(name, Type.DefaultLookup);
+        public abstract EventInfo? GetEvent(string name, BindingFlags bindingAttr);
 
         public virtual EventInfo[] GetEvents() => GetEvents(Type.DefaultLookup);
         public abstract EventInfo[] GetEvents(BindingFlags bindingAttr);
 
-        public FieldInfo GetField(string name) => GetField(name, Type.DefaultLookup);
-        public abstract FieldInfo GetField(string name, BindingFlags bindingAttr);
+        public FieldInfo? GetField(string name) => GetField(name, Type.DefaultLookup);
+        public abstract FieldInfo? GetField(string name, BindingFlags bindingAttr);
 
         public FieldInfo[] GetFields() => GetFields(Type.DefaultLookup);
         public abstract FieldInfo[] GetFields(BindingFlags bindingAttr);
@@ -156,18 +157,18 @@ public ConstructorInfo GetConstructor(BindingFlags bindingAttr, Binder binder, C
         public MemberInfo[] GetMembers() => GetMembers(Type.DefaultLookup);
         public abstract MemberInfo[] GetMembers(BindingFlags bindingAttr);
 
-        public MethodInfo GetMethod(string name) => GetMethod(name, Type.DefaultLookup);
-        public MethodInfo GetMethod(string name, BindingFlags bindingAttr)
+        public MethodInfo? GetMethod(string name) => GetMethod(name, Type.DefaultLookup);
+        public MethodInfo? GetMethod(string name, BindingFlags bindingAttr)
         {
             if (name == null)
                 throw new ArgumentNullException(nameof(name));
             return GetMethodImpl(name, bindingAttr, null, CallingConventions.Any, null, null);
         }
 
-        public MethodInfo GetMethod(string name, Type[] types) => GetMethod(name, types, null);
-        public MethodInfo GetMethod(string name, Type[] types, ParameterModifier[] modifiers) => GetMethod(name, Type.DefaultLookup, null, types, modifiers);
-        public MethodInfo GetMethod(string name, BindingFlags bindingAttr, Binder binder, Type[] types, ParameterModifier[] modifiers) => GetMethod(name, bindingAttr, binder, CallingConventions.Any, types, modifiers);
-        public MethodInfo GetMethod(string name, BindingFlags bindingAttr, Binder binder, CallingConventions callConvention, Type[] types, ParameterModifier[] modifiers)
+        public MethodInfo? GetMethod(string name, Type[] types) => GetMethod(name, types, null);
+        public MethodInfo? GetMethod(string name, Type[] types, ParameterModifier[]? modifiers) => GetMethod(name, Type.DefaultLookup, null, types, modifiers);
+        public MethodInfo? GetMethod(string name, BindingFlags bindingAttr, Binder? binder, Type[] types, ParameterModifier[]? modifiers) => GetMethod(name, bindingAttr, binder, CallingConventions.Any, types, modifiers);
+        public MethodInfo? GetMethod(string name, BindingFlags bindingAttr, Binder? binder, CallingConventions callConvention, Type[] types, ParameterModifier[]? modifiers)
         {
             if (name == null)
                 throw new ArgumentNullException(nameof(name));
@@ -181,12 +182,12 @@ public MethodInfo GetMethod(string name, BindingFlags bindingAttr, Binder binder
             return GetMethodImpl(name, bindingAttr, binder, callConvention, types, modifiers);
         }
 
-        protected abstract MethodInfo GetMethodImpl(string name, BindingFlags bindingAttr, Binder binder, CallingConventions callConvention, Type[] types, ParameterModifier[] modifiers);
+        protected abstract MethodInfo? GetMethodImpl(string name, BindingFlags bindingAttr, Binder? binder, CallingConventions callConvention, Type[]? types, ParameterModifier[]? modifiers);
 
-        public MethodInfo GetMethod(string name, int genericParameterCount, Type[] types) => GetMethod(name, genericParameterCount, types, null);
-        public MethodInfo GetMethod(string name, int genericParameterCount, Type[] types, ParameterModifier[] modifiers) => GetMethod(name, genericParameterCount, Type.DefaultLookup, null, types, modifiers);
-        public MethodInfo GetMethod(string name, int genericParameterCount, BindingFlags bindingAttr, Binder binder, Type[] types, ParameterModifier[] modifiers) => GetMethod(name, genericParameterCount, bindingAttr, binder, CallingConventions.Any, types, modifiers);
-        public MethodInfo GetMethod(string name, int genericParameterCount, BindingFlags bindingAttr, Binder binder, CallingConventions callConvention, Type[] types, ParameterModifier[] modifiers)
+        public MethodInfo? GetMethod(string name, int genericParameterCount, Type[] types) => GetMethod(name, genericParameterCount, types, null);
+        public MethodInfo? GetMethod(string name, int genericParameterCount, Type[] types, ParameterModifier[]? modifiers) => GetMethod(name, genericParameterCount, Type.DefaultLookup, null, types, modifiers);
+        public MethodInfo? GetMethod(string name, int genericParameterCount, BindingFlags bindingAttr, Binder? binder, Type[] types, ParameterModifier[]? modifiers) => GetMethod(name, genericParameterCount, bindingAttr, binder, CallingConventions.Any, types, modifiers);
+        public MethodInfo? GetMethod(string name, int genericParameterCount, BindingFlags bindingAttr, Binder? binder, CallingConventions callConvention, Type[] types, ParameterModifier[]? modifiers)
         {
             if (name == null)
                 throw new ArgumentNullException(nameof(name));
@@ -202,26 +203,26 @@ public MethodInfo GetMethod(string name, int genericParameterCount, BindingFlags
             return GetMethodImpl(name, genericParameterCount, bindingAttr, binder, callConvention, types, modifiers);
         }
 
-        protected virtual MethodInfo GetMethodImpl(string name, int genericParameterCount, BindingFlags bindingAttr, Binder binder, CallingConventions callConvention, Type[] types, ParameterModifier[] modifiers) => throw new NotSupportedException();
+        protected virtual MethodInfo? GetMethodImpl(string name, int genericParameterCount, BindingFlags bindingAttr, Binder? binder, CallingConventions callConvention, Type[]? types, ParameterModifier[]? modifiers) => throw new NotSupportedException();
 
         public MethodInfo[] GetMethods() => GetMethods(Type.DefaultLookup);
         public abstract MethodInfo[] GetMethods(BindingFlags bindingAttr);
 
-        public Type GetNestedType(string name) => GetNestedType(name, Type.DefaultLookup);
-        public abstract Type GetNestedType(string name, BindingFlags bindingAttr);
+        public Type? GetNestedType(string name) => GetNestedType(name, Type.DefaultLookup);
+        public abstract Type? GetNestedType(string name, BindingFlags bindingAttr);
 
         public Type[] GetNestedTypes() => GetNestedTypes(Type.DefaultLookup);
         public abstract Type[] GetNestedTypes(BindingFlags bindingAttr);
 
-        public PropertyInfo GetProperty(string name) => GetProperty(name, Type.DefaultLookup);
-        public PropertyInfo GetProperty(string name, BindingFlags bindingAttr)
+        public PropertyInfo? GetProperty(string name) => GetProperty(name, Type.DefaultLookup);
+        public PropertyInfo? GetProperty(string name, BindingFlags bindingAttr)
         {
             if (name == null)
                 throw new ArgumentNullException(nameof(name));
             return GetPropertyImpl(name, bindingAttr, null, null, null, null);
         }
 
-        public PropertyInfo GetProperty(string name, Type returnType)
+        public PropertyInfo? GetProperty(string name, Type returnType)
         {
             if (name == null)
                 throw new ArgumentNullException(nameof(name));
@@ -230,10 +231,10 @@ public PropertyInfo GetProperty(string name, Type returnType)
             return GetPropertyImpl(name, Type.DefaultLookup, null, returnType, null, null);
         }
 
-        public PropertyInfo GetProperty(string name, Type[] types) => GetProperty(name, null, types);
-        public PropertyInfo GetProperty(string name, Type returnType, Type[] types) => GetProperty(name, returnType, types, null);
-        public PropertyInfo GetProperty(string name, Type returnType, Type[] types, ParameterModifier[] modifiers) => GetProperty(name, Type.DefaultLookup, null, returnType, types, modifiers);
-        public PropertyInfo GetProperty(string name, BindingFlags bindingAttr, Binder binder, Type returnType, Type[] types, ParameterModifier[] modifiers)
+        public PropertyInfo? GetProperty(string name, Type[] types) => GetProperty(name, null, types);
+        public PropertyInfo? GetProperty(string name, Type? returnType, Type[] types) => GetProperty(name, returnType, types, null);
+        public PropertyInfo? GetProperty(string name, Type? returnType, Type[] types, ParameterModifier[]? modifiers) => GetProperty(name, Type.DefaultLookup, null, returnType, types, modifiers);
+        public PropertyInfo? GetProperty(string name, BindingFlags bindingAttr, Binder? binder, Type? returnType, Type[] types, ParameterModifier[]? modifiers)
         {
             if (name == null)
                 throw new ArgumentNullException(nameof(name));
@@ -242,7 +243,7 @@ public PropertyInfo GetProperty(string name, BindingFlags bindingAttr, Binder bi
             return GetPropertyImpl(name, bindingAttr, binder, returnType, types, modifiers);
         }
 
-        protected abstract PropertyInfo GetPropertyImpl(string name, BindingFlags bindingAttr, Binder binder, Type returnType, Type[] types, ParameterModifier[] modifiers);
+        protected abstract PropertyInfo? GetPropertyImpl(string name, BindingFlags bindingAttr, Binder? binder, Type? returnType, Type[]? types, ParameterModifier[]? modifiers);
 
         public PropertyInfo[] GetProperties() => GetProperties(Type.DefaultLookup);
         public abstract PropertyInfo[] GetProperties(BindingFlags bindingAttr);
@@ -273,7 +274,7 @@ public static Type[] GetTypeArray(object[] args)
             return cls;
         }
 
-        public static TypeCode GetTypeCode(Type type)
+        public static TypeCode GetTypeCode(Type? type)
         {
             if (type == null)
                 return TypeCode.Empty;
@@ -292,31 +293,31 @@ protected virtual TypeCode GetTypeCodeImpl()
 
         public static Type GetTypeFromCLSID(Guid clsid) => GetTypeFromCLSID(clsid, null, throwOnError: false);
         public static Type GetTypeFromCLSID(Guid clsid, bool throwOnError) => GetTypeFromCLSID(clsid, null, throwOnError: throwOnError);
-        public static Type GetTypeFromCLSID(Guid clsid, string server) => GetTypeFromCLSID(clsid, server, throwOnError: false);
+        public static Type GetTypeFromCLSID(Guid clsid, string? server) => GetTypeFromCLSID(clsid, server, throwOnError: false);
 
         public static Type GetTypeFromProgID(string progID) => GetTypeFromProgID(progID, null, throwOnError: false);
         public static Type GetTypeFromProgID(string progID, bool throwOnError) => GetTypeFromProgID(progID, null, throwOnError: throwOnError);
-        public static Type GetTypeFromProgID(string progID, string server) => GetTypeFromProgID(progID, server, throwOnError: false);
+        public static Type GetTypeFromProgID(string progID, string? server) => GetTypeFromProgID(progID, server, throwOnError: false);
 
-        public abstract Type BaseType { get; }
+        public abstract Type? BaseType { get; }
 
         [DebuggerHidden]
         [DebuggerStepThrough]
-        public object InvokeMember(string name, BindingFlags invokeAttr, Binder binder, object target, object[] args) => InvokeMember(name, invokeAttr, binder, target, args, null, null, null);
+        public object? InvokeMember(string name, BindingFlags invokeAttr, Binder? binder, object? target, object[]? args) => InvokeMember(name, invokeAttr, binder, target, args, null, null, null);
 
         [DebuggerHidden]
         [DebuggerStepThrough]
-        public object InvokeMember(string name, BindingFlags invokeAttr, Binder binder, object target, object[] args, CultureInfo culture) => InvokeMember(name, invokeAttr, binder, target, args, null, culture, null);
-        public abstract object InvokeMember(string name, BindingFlags invokeAttr, Binder binder, object target, object[] args, ParameterModifier[] modifiers, CultureInfo culture, string[] namedParameters);
+        public object? InvokeMember(string name, BindingFlags invokeAttr, Binder? binder, object? target, object[]? args, CultureInfo? culture) => InvokeMember(name, invokeAttr, binder, target, args, null, culture, null);
+        public abstract object? InvokeMember(string name, BindingFlags invokeAttr, Binder? binder, object? target, object[]? args, ParameterModifier[]? modifiers, CultureInfo? culture, string[]? namedParameters);
 
-        public Type GetInterface(string name) => GetInterface(name, ignoreCase: false);
-        public abstract Type GetInterface(string name, bool ignoreCase);
+        public Type? GetInterface(string name) => GetInterface(name, ignoreCase: false);
+        public abstract Type? GetInterface(string name, bool ignoreCase);
         public abstract Type[] GetInterfaces();
 
         public virtual InterfaceMapping GetInterfaceMap(Type interfaceType) { throw new NotSupportedException(SR.NotSupported_SubclassOverride); }
 
-        public virtual bool IsInstanceOfType(object o) => o == null ? false : IsAssignableFrom(o.GetType());
-        public virtual bool IsEquivalentTo(Type other) => this == other;
+        public virtual bool IsInstanceOfType(object? o) => o == null ? false : IsAssignableFrom(o.GetType());
+        public virtual bool IsEquivalentTo(Type? other) => this == other;
 
         public virtual Type GetEnumUnderlyingType()
         {
@@ -356,7 +357,7 @@ public static Type MakeGenericMethodParameter(int position)
 
         public override string ToString() => "Type: " + Name;  // Why do we add the "Type: " prefix?
 
-        public override bool Equals(object o) => o == null ? false : Equals(o as Type);
+        public override bool Equals(object? o) => o == null ? false : Equals(o as Type);
         public override int GetHashCode()
         {
             Type systemType = UnderlyingSystemType;
@@ -364,7 +365,7 @@ public override int GetHashCode()
                 return systemType.GetHashCode();
             return base.GetHashCode();
         }
-        public virtual bool Equals(Type o) => o == null ? false : object.ReferenceEquals(this.UnderlyingSystemType, o.UnderlyingSystemType);
+        public virtual bool Equals(Type? o) => o == null ? false : object.ReferenceEquals(this.UnderlyingSystemType, o.UnderlyingSystemType);
 
         public static Type ReflectionOnlyGetType(string typeName, bool throwIfNotFound, bool ignoreCase) { throw new PlatformNotSupportedException(SR.PlatformNotSupported_ReflectionOnly); }
 
@@ -375,13 +376,13 @@ public static Binder DefaultBinder
                 if (s_defaultBinder == null)
                 {
                     DefaultBinder binder = new DefaultBinder();
-                    Interlocked.CompareExchange<Binder>(ref s_defaultBinder, binder, null);
+                    Interlocked.CompareExchange<Binder?>(ref s_defaultBinder, binder, null);
                 }
-                return s_defaultBinder;
+                return s_defaultBinder!;
             }
         }
 
-        private static volatile Binder s_defaultBinder;
+        private static volatile Binder? s_defaultBinder;
 
         public static readonly char Delimiter = '.';
         public static readonly Type[] EmptyTypes = Array.Empty<Type>();