diff --git a/src/Common/src/CoreLib/System/Array.cs b/src/Common/src/CoreLib/System/Array.cs
index 592cd55bd307..6bda3a65733b 100644
--- a/src/Common/src/CoreLib/System/Array.cs
+++ b/src/Common/src/CoreLib/System/Array.cs
@@ -384,7 +384,7 @@ int IStructuralEquatable.GetHashCode(IEqualityComparer comparer)
 
             for (int i = (this.Length >= 8 ? this.Length - 8 : 0); i < this.Length; i++)
             {
-                ret = CombineHashCodes(ret, comparer!.GetHashCode(GetValue(i))); // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/538
+                ret = CombineHashCodes(ret, comparer!.GetHashCode(GetValue(i)!)); // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/538
             }
 
             return ret;
@@ -1592,7 +1592,7 @@ public static void Sort<T>(T[] array, Comparison<T> comparison)
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.comparison);
             }
 
-            ArraySortHelper<T>.Sort(array!, 0, array!.Length, comparison); // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/538
+            ArraySortHelper<T>.Sort(array!, 0, array!.Length, comparison!); // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/538
         }
 
         public static bool TrueForAll<T>(T[] array, Predicate<T> match)
diff --git a/src/Common/src/CoreLib/System/Collections/DictionaryEntry.cs b/src/Common/src/CoreLib/System/Collections/DictionaryEntry.cs
index 187301a08f49..b2070224ed07 100644
--- a/src/Common/src/CoreLib/System/Collections/DictionaryEntry.cs
+++ b/src/Common/src/CoreLib/System/Collections/DictionaryEntry.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.ComponentModel;
 
 namespace System.Collections
@@ -13,11 +14,11 @@ namespace System.Collections
     public struct DictionaryEntry
     {
         private object _key; // Do not rename (binary serialization)
-        private object _value; // Do not rename (binary serialization)
+        private object? _value; // Do not rename (binary serialization)
 
         // Constructs a new DictionaryEnumerator by setting the Key
         // and Value fields appropriately.
-        public DictionaryEntry(object key, object value)
+        public DictionaryEntry(object key, object? value)
         {
             _key = key;
             _value = value;
@@ -36,7 +37,7 @@ public object Key
             }
         }
 
-        public object Value
+        public object? Value
         {
             get
             {
@@ -50,7 +51,7 @@ public object Value
         }
 
         [EditorBrowsable(EditorBrowsableState.Never)]
-        public void Deconstruct(out object key, out object value)
+        public void Deconstruct(out object key, out object? value)
         {
             key = Key;
             value = Value;
diff --git a/src/Common/src/CoreLib/System/Collections/Generic/ArraySortHelper.cs b/src/Common/src/CoreLib/System/Collections/Generic/ArraySortHelper.cs
index 03b9865044f6..7d7d94580a8c 100644
--- a/src/Common/src/CoreLib/System/Collections/Generic/ArraySortHelper.cs
+++ b/src/Common/src/CoreLib/System/Collections/Generic/ArraySortHelper.cs
@@ -13,6 +13,7 @@
 ** 
 ===========================================================*/
 
+#nullable enable
 using System.Diagnostics;
 using System.Runtime.CompilerServices;
 
@@ -38,7 +39,7 @@ internal static int FloorLog2PlusOne(int n)
             return result;
         }
 
-        internal static void ThrowOrIgnoreBadComparer(object comparer)
+        internal static void ThrowOrIgnoreBadComparer(object? comparer)
         {
             throw new ArgumentException(SR.Format(SR.Arg_BogusIComparer, comparer));
         }
@@ -48,7 +49,7 @@ internal partial class ArraySortHelper<T>
     {
         #region IArraySortHelper<T> Members
 
-        public void Sort(T[] keys, int index, int length, IComparer<T> comparer)
+        public void Sort(T[] keys, int index, int length, IComparer<T>? comparer)
         {
             Debug.Assert(keys != null, "Check the arguments in the caller!");
             Debug.Assert(index >= 0 && length >= 0 && (keys.Length - index >= length), "Check the arguments in the caller!");
@@ -74,7 +75,7 @@ public void Sort(T[] keys, int index, int length, IComparer<T> comparer)
             }
         }
 
-        public int BinarySearch(T[] array, int index, int length, T value, IComparer<T> comparer)
+        public int BinarySearch(T[] array, int index, int length, T value, IComparer<T>? comparer)
         {
             try
             {
@@ -335,7 +336,7 @@ internal partial class GenericArraySortHelper<T>
 
         #region IArraySortHelper<T> Members
 
-        public void Sort(T[] keys, int index, int length, IComparer<T> comparer)
+        public void Sort(T[] keys, int index, int length, IComparer<T>? comparer)
         {
             Debug.Assert(keys != null, "Check the arguments in the caller!");
             Debug.Assert(index >= 0 && length >= 0 && (keys.Length - index >= length), "Check the arguments in the caller!");
@@ -361,7 +362,7 @@ public void Sort(T[] keys, int index, int length, IComparer<T> comparer)
             }
         }
 
-        public int BinarySearch(T[] array, int index, int length, T value, IComparer<T> comparer)
+        public int BinarySearch(T[] array, int index, int length, T value, IComparer<T>? comparer)
         {
             Debug.Assert(array != null, "Check the arguments in the caller!");
             Debug.Assert(index >= 0 && length >= 0 && (array.Length - index >= length), "Check the arguments in the caller!");
@@ -624,7 +625,7 @@ private static void InsertionSort(T[] keys, int lo, int hi)
 
     internal partial class ArraySortHelper<TKey, TValue>
     {
-        public void Sort(TKey[] keys, TValue[] values, int index, int length, IComparer<TKey> comparer)
+        public void Sort(TKey[] keys, TValue[] values, int index, int length, IComparer<TKey>? comparer)
         {
             Debug.Assert(keys != null, "Check the arguments in the caller!");  // Precondition on interface method
             Debug.Assert(values != null, "Check the arguments in the caller!");
@@ -871,7 +872,7 @@ private static void InsertionSort(TKey[] keys, TValue[] values, int lo, int hi,
     internal partial class GenericArraySortHelper<TKey, TValue>
         where TKey : IComparable<TKey>
     {
-        public void Sort(TKey[] keys, TValue[] values, int index, int length, IComparer<TKey> comparer)
+        public void Sort(TKey[] keys, TValue[] values, int index, int length, IComparer<TKey>? comparer)
         {
             Debug.Assert(keys != null, "Check the arguments in the caller!");
             Debug.Assert(index >= 0 && length >= 0 && (keys.Length - index >= length), "Check the arguments in the caller!");
diff --git a/src/Common/src/CoreLib/System/Collections/Generic/Comparer.cs b/src/Common/src/CoreLib/System/Collections/Generic/Comparer.cs
index f7c6cf81dd65..aa36769a4315 100644
--- a/src/Common/src/CoreLib/System/Collections/Generic/Comparer.cs
+++ b/src/Common/src/CoreLib/System/Collections/Generic/Comparer.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;
 using System.Runtime.Serialization;
 
@@ -21,9 +22,9 @@ public static Comparer<T> Create(Comparison<T> comparison)
             return new ComparisonComparer<T>(comparison);
         }
 
-        public abstract int Compare(T x, T y);
+        public abstract int Compare(T x, T y); // TODO-NULLABLE-GENERIC: x and y must be marked as nullable
 
-        int IComparer.Compare(object x, object y)
+        int IComparer.Compare(object? x, object? y)
         {
             if (x == null) return y == null ? 0 : -1;
             if (y == null) return 1;
@@ -58,7 +59,7 @@ public override int Compare(T x, T y)
     // Needs to be public to support binary serialization compatibility
     public sealed partial class GenericComparer<T> : Comparer<T> where T : IComparable<T>
     {
-        public override int Compare(T x, T y)
+        public override int Compare(T x, T y) // TODO-NULLABLE-GENERIC: x and y must be marked as nullable
         {
             if (x != null)
             {
@@ -70,7 +71,7 @@ public override int Compare(T x, T y)
         }
 
         // Equals method for the comparer itself. 
-        public override bool Equals(object obj) =>
+        public override bool Equals(object? obj) =>
             obj != null && GetType() == obj.GetType();
 
         public override int GetHashCode() =>
@@ -94,7 +95,7 @@ public override int Compare(T? x, T? y)
         }
 
         // Equals method for the comparer itself. 
-        public override bool Equals(object obj) =>
+        public override bool Equals(object? obj) =>
             obj != null && GetType() == obj.GetType();
 
         public override int GetHashCode() =>
@@ -106,13 +107,13 @@ public override int GetHashCode() =>
     // Needs to be public to support binary serialization compatibility
     public sealed partial class ObjectComparer<T> : Comparer<T>
     {
-        public override int Compare(T x, T y)
+        public override int Compare(T x, T y) // TODO-NULLABLE-GENERIC: x and y must be marked as nullable
         {
             return System.Collections.Comparer.Default.Compare(x, y);
         }
 
         // Equals method for the comparer itself. 
-        public override bool Equals(object obj) =>
+        public override bool Equals(object? obj) =>
             obj != null && GetType() == obj.GetType();
 
         public override int GetHashCode() =>
@@ -130,7 +131,7 @@ private EnumComparer(SerializationInfo info, StreamingContext context) { }
         // public override int Compare(T x, T y) is runtime-specific
 
         // Equals method for the comparer itself. 
-        public override bool Equals(object obj) =>
+        public override bool Equals(object? obj) =>
             obj != null && GetType() == obj.GetType();
 
         public override int GetHashCode() =>
diff --git a/src/Common/src/CoreLib/System/Collections/Generic/Dictionary.cs b/src/Common/src/CoreLib/System/Collections/Generic/Dictionary.cs
index 0a36cb5f7681..27442029addb 100644
--- a/src/Common/src/CoreLib/System/Collections/Generic/Dictionary.cs
+++ b/src/Common/src/CoreLib/System/Collections/Generic/Dictionary.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;
 using System.Runtime.Serialization;
@@ -33,7 +34,7 @@ internal enum InsertionBehavior : byte
     [DebuggerDisplay("Count = {Count}")]
     [Serializable]
     [TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
-    public class Dictionary<TKey, TValue> : IDictionary<TKey, TValue>, IDictionary, IReadOnlyDictionary<TKey, TValue>, ISerializable, IDeserializationCallback
+    public class Dictionary<TKey, TValue> : IDictionary<TKey, TValue>, IDictionary, IReadOnlyDictionary<TKey, TValue>, ISerializable, IDeserializationCallback where TKey : object
     {
         private struct Entry
         {
@@ -46,15 +47,15 @@ private struct Entry
             public TValue value;         // Value of entry
         }
 
-        private int[] _buckets;
-        private Entry[] _entries;
+        private int[]? _buckets;
+        private Entry[]? _entries;
         private int _count;
         private int _freeList;
         private int _freeCount;
         private int _version;
-        private IEqualityComparer<TKey> _comparer;
-        private KeyCollection _keys;
-        private ValueCollection _values;
+        private IEqualityComparer<TKey>? _comparer;
+        private KeyCollection? _keys;
+        private ValueCollection? _values;
         private const int StartOfFreeList = -3;
 
         // constants for serialization
@@ -67,9 +68,9 @@ public Dictionary() : this(0, null) { }
 
         public Dictionary(int capacity) : this(capacity, null) { }
 
-        public Dictionary(IEqualityComparer<TKey> comparer) : this(0, comparer) { }
+        public Dictionary(IEqualityComparer<TKey>? comparer) : this(0, comparer) { }
 
-        public Dictionary(int capacity, IEqualityComparer<TKey> comparer)
+        public Dictionary(int capacity, IEqualityComparer<TKey>? comparer)
         {
             if (capacity < 0) ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.capacity);
             if (capacity > 0) Initialize(capacity);
@@ -87,7 +88,7 @@ public Dictionary(int capacity, IEqualityComparer<TKey> comparer)
 
         public Dictionary(IDictionary<TKey, TValue> dictionary) : this(dictionary, null) { }
 
-        public Dictionary(IDictionary<TKey, TValue> dictionary, IEqualityComparer<TKey> comparer) :
+        public Dictionary(IDictionary<TKey, TValue> dictionary, IEqualityComparer<TKey>? comparer) :
             this(dictionary != null ? dictionary.Count : 0, comparer)
         {
             if (dictionary == null)
@@ -99,14 +100,14 @@ public Dictionary(IDictionary<TKey, TValue> dictionary, IEqualityComparer<TKey>
             // avoid the enumerator allocation and overhead by looping through the entries array directly.
             // We only do this when dictionary is Dictionary<TKey,TValue> and not a subclass, to maintain
             // back-compat with subclasses that may have overridden the enumerator behavior.
-            if (dictionary.GetType() == typeof(Dictionary<TKey, TValue>))
+            if (dictionary!.GetType() == typeof(Dictionary<TKey, TValue>)) // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/538
             {
                 Dictionary<TKey, TValue> d = (Dictionary<TKey, TValue>)dictionary;
                 int count = d._count;
-                Entry[] entries = d._entries;
+                Entry[]? entries = d._entries;
                 for (int i = 0; i < count; i++)
                 {
-                    if (entries[i].next >= -1)
+                    if (entries![i].next >= -1)
                     {
                         Add(entries[i].key, entries[i].value);
                     }
@@ -122,7 +123,7 @@ public Dictionary(IDictionary<TKey, TValue> dictionary, IEqualityComparer<TKey>
 
         public Dictionary(IEnumerable<KeyValuePair<TKey, TValue>> collection) : this(collection, null) { }
 
-        public Dictionary(IEnumerable<KeyValuePair<TKey, TValue>> collection, IEqualityComparer<TKey> comparer) :
+        public Dictionary(IEnumerable<KeyValuePair<TKey, TValue>> collection, IEqualityComparer<TKey>? comparer) :
             this((collection as ICollection<KeyValuePair<TKey, TValue>>)?.Count ?? 0, comparer)
         {
             if (collection == null)
@@ -130,7 +131,7 @@ public Dictionary(IEnumerable<KeyValuePair<TKey, TValue>> collection, IEqualityC
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.collection);
             }
 
-            foreach (KeyValuePair<TKey, TValue> pair in collection)
+            foreach (KeyValuePair<TKey, TValue> pair in collection!) // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/538
             {
                 Add(pair.Key, pair.Value);
             }
@@ -216,9 +217,9 @@ public TValue this[TKey key]
             get
             {
                 int i = FindEntry(key);
-                if (i >= 0) return _entries[i].value;
+                if (i >= 0) return _entries![i].value;
                 ThrowHelper.ThrowKeyNotFoundException(key);
-                return default;
+                return default!; // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/538 (annotating ThrowHelper removes this return statement).
             }
             set
             {
@@ -239,7 +240,7 @@ void ICollection<KeyValuePair<TKey, TValue>>.Add(KeyValuePair<TKey, TValue> keyV
         bool ICollection<KeyValuePair<TKey, TValue>>.Contains(KeyValuePair<TKey, TValue> keyValuePair)
         {
             int i = FindEntry(keyValuePair.Key);
-            if (i >= 0 && EqualityComparer<TValue>.Default.Equals(_entries[i].value, keyValuePair.Value))
+            if (i >= 0 && EqualityComparer<TValue>.Default.Equals(_entries![i].value, keyValuePair.Value))
             {
                 return true;
             }
@@ -249,7 +250,7 @@ bool ICollection<KeyValuePair<TKey, TValue>>.Contains(KeyValuePair<TKey, TValue>
         bool ICollection<KeyValuePair<TKey, TValue>>.Remove(KeyValuePair<TKey, TValue> keyValuePair)
         {
             int i = FindEntry(keyValuePair.Key);
-            if (i >= 0 && EqualityComparer<TValue>.Default.Equals(_entries[i].value, keyValuePair.Value))
+            if (i >= 0 && EqualityComparer<TValue>.Default.Equals(_entries![i].value, keyValuePair.Value))
             {
                 Remove(keyValuePair.Key);
                 return true;
@@ -262,6 +263,9 @@ public void Clear()
             int count = _count;
             if (count > 0)
             {
+                Debug.Assert(_buckets != null, "_buckets should be non-null");
+                Debug.Assert(_entries != null, "_entries should be non-null");
+
                 Array.Clear(_buckets, 0, _buckets.Length);
 
                 _count = 0;
@@ -276,22 +280,22 @@ public bool ContainsKey(TKey key)
 
         public bool ContainsValue(TValue value)
         {
-            Entry[] entries = _entries;
+            Entry[]? entries = _entries;
             if (value == null)
             {
                 for (int i = 0; i < _count; i++)
                 {
-                    if (entries[i].next >= -1 && entries[i].value == null) return true;
+                    if (entries![i].next >= -1 && entries[i].value == null) return true;
                 }
             }
             else
             {
-                if (default(TValue) != null)
+                if (default(TValue)! != null) // TODO-NULLABLE-GENERIC: https://github.com/dotnet/roslyn/issues/34757
                 {
                     // ValueType: Devirtualize with EqualityComparer<TValue>.Default intrinsic
                     for (int i = 0; i < _count; i++)
                     {
-                        if (entries[i].next >= -1 && EqualityComparer<TValue>.Default.Equals(entries[i].value, value)) return true;
+                        if (entries![i].next >= -1 && EqualityComparer<TValue>.Default.Equals(entries[i].value, value)) return true;
                     }
                 }
                 else
@@ -302,7 +306,7 @@ public bool ContainsValue(TValue value)
                     EqualityComparer<TValue> defaultComparer = EqualityComparer<TValue>.Default;
                     for (int i = 0; i < _count; i++)
                     {
-                        if (entries[i].next >= -1 && defaultComparer.Equals(entries[i].value, value)) return true;
+                        if (entries![i].next >= -1 && defaultComparer.Equals(entries[i].value, value)) return true;
                     }
                 }
             }
@@ -316,7 +320,7 @@ private void CopyTo(KeyValuePair<TKey, TValue>[] array, int index)
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
             }
 
-            if ((uint)index > (uint)array.Length)
+            if ((uint)index > (uint)array!.Length) // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/538
             {
                 ThrowHelper.ThrowIndexArgumentOutOfRange_NeedNonNegNumException();
             }
@@ -327,10 +331,10 @@ private void CopyTo(KeyValuePair<TKey, TValue>[] array, int index)
             }
 
             int count = _count;
-            Entry[] entries = _entries;
+            Entry[]? entries = _entries;
             for (int i = 0; i < count; i++)
             {
-                if (entries[i].next >= -1)
+                if (entries![i].next >= -1)
                 {
                     array[index++] = new KeyValuePair<TKey, TValue>(entries[i].key, entries[i].value);
                 }
@@ -350,7 +354,7 @@ public virtual void GetObjectData(SerializationInfo info, StreamingContext conte
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.info);
             }
 
-            info.AddValue(VersionName, _version);
+            info!.AddValue(VersionName, _version); // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/538
             info.AddValue(ComparerName, _comparer ?? EqualityComparer<TKey>.Default, typeof(IEqualityComparer<TKey>));
             info.AddValue(HashSizeName, _buckets == null ? 0 : _buckets.Length); // This is the length of the bucket array
 
@@ -370,18 +374,19 @@ private int FindEntry(TKey key)
             }
 
             int i = -1;
-            int[] buckets = _buckets;
-            Entry[] entries = _entries;
+            int[]? buckets = _buckets;
+            Entry[]? entries = _entries;
             int collisionCount = 0;
             if (buckets != null)
             {
-                IEqualityComparer<TKey> comparer = _comparer;
+                Debug.Assert(entries != null, "expected entries to be != null");
+                IEqualityComparer<TKey>? comparer = _comparer;
                 if (comparer == null)
                 {
-                    uint hashCode = (uint)key.GetHashCode();
+                    uint hashCode = (uint)key!.GetHashCode(); // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/26761
                     // Value in _buckets is 1-based
                     i = buckets[hashCode % (uint)buckets.Length] - 1;
-                    if (default(TKey) != null)
+                    if (default(TKey)! != null) // TODO-NULLABLE-GENERIC: https://github.com/dotnet/roslyn/issues/34757
                     {
                         // ValueType: Devirtualize with EqualityComparer<TValue>.Default intrinsic
                         do
@@ -482,19 +487,20 @@ private bool TryInsert(TKey key, TValue value, InsertionBehavior behavior)
                 Initialize(0);
             }
 
-            Entry[] entries = _entries;
-            IEqualityComparer<TKey> comparer = _comparer;
+            Entry[]? entries = _entries;
+            Debug.Assert(entries != null, "expected entries to be non-null");
 
-            uint hashCode = (uint)((comparer == null) ? key.GetHashCode() : comparer.GetHashCode(key));
+            IEqualityComparer<TKey>? comparer = _comparer;
+            uint hashCode = (uint)((comparer == null) ? key!.GetHashCode() : comparer.GetHashCode(key)); // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/26761
 
             int collisionCount = 0;
-            ref int bucket = ref _buckets[hashCode % (uint)_buckets.Length];
+            ref int bucket = ref _buckets![hashCode % (uint)_buckets.Length]; // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/26761 (Initialize inits sets _buckets to non null)
             // Value in _buckets is 1-based
             int i = bucket - 1;
 
             if (comparer == null)
             {
-                if (default(TKey) != null)
+                if (default(TKey)! != null) // TODO-NULLABLE-GENERIC: https://github.com/dotnet/roslyn/issues/34757
                 {
                     // ValueType: Devirtualize with EqualityComparer<TValue>.Default intrinsic
                     do
@@ -637,7 +643,7 @@ private bool TryInsert(TKey key, TValue value, InsertionBehavior behavior)
                 entries = _entries;
             }
 
-            ref Entry entry = ref entries[index];
+            ref Entry entry = ref entries![index];
 
             if (updateFreeList)
             {
@@ -655,7 +661,7 @@ private bool TryInsert(TKey key, TValue value, InsertionBehavior behavior)
             _version++;
 
             // Value types never rehash
-            if (default(TKey) == null && collisionCount > HashHelpers.HashCollisionThreshold && comparer is NonRandomizedStringEqualityComparer)
+            if (default(TKey)! == null && collisionCount > HashHelpers.HashCollisionThreshold && comparer is NonRandomizedStringEqualityComparer) // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/34757
             {
                 // If we hit the collision threshold we'll need to switch to the comparer which is using randomized string hashing
                 // i.e. EqualityComparer<string>.Default.
@@ -679,13 +685,13 @@ public virtual void OnDeserialization(object sender)
 
             int realVersion = siInfo.GetInt32(VersionName);
             int hashsize = siInfo.GetInt32(HashSizeName);
-            _comparer = (IEqualityComparer<TKey>)siInfo.GetValue(ComparerName, typeof(IEqualityComparer<TKey>));
+            _comparer = (IEqualityComparer<TKey>)siInfo.GetValue(ComparerName, typeof(IEqualityComparer<TKey>))!; // When serialized if comparer is null, we use the default.
 
             if (hashsize != 0)
             {
                 Initialize(hashsize);
 
-                KeyValuePair<TKey, TValue>[] array = (KeyValuePair<TKey, TValue>[])
+                KeyValuePair<TKey, TValue>[]? array = (KeyValuePair<TKey, TValue>[]?)
                     siInfo.GetValue(KeyValuePairsName, typeof(KeyValuePair<TKey, TValue>[]));
 
                 if (array == null)
@@ -693,7 +699,7 @@ public virtual void OnDeserialization(object sender)
                     ThrowHelper.ThrowSerializationException(ExceptionResource.Serialization_MissingKeys);
                 }
 
-                for (int i = 0; i < array.Length; i++)
+                for (int i = 0; i < array!.Length; i++) // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/538
                 {
                     if (array[i].Key == null)
                     {
@@ -717,7 +723,8 @@ private void Resize()
         private void Resize(int newSize, bool forceNewHashCodes)
         {
             // Value types never rehash
-            Debug.Assert(!forceNewHashCodes || default(TKey) == null);
+            Debug.Assert(!forceNewHashCodes || default(TKey)! == null); // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/34757
+            Debug.Assert(_entries != null, "_entries should be non-null");
             Debug.Assert(newSize >= _entries.Length);
 
             int[] buckets = new int[newSize];
@@ -726,7 +733,7 @@ private void Resize(int newSize, bool forceNewHashCodes)
             int count = _count;
             Array.Copy(_entries, 0, entries, 0, count);
 
-            if (default(TKey) == null && forceNewHashCodes)
+            if (default(TKey)! == null && forceNewHashCodes) // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/34757
             {
                 for (int i = 0; i < count; i++)
                 {
@@ -764,12 +771,13 @@ public bool Remove(TKey key)
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.key);
             }
 
-            int[] buckets = _buckets;
-            Entry[] entries = _entries;
+            int[]? buckets = _buckets;
+            Entry[]? entries = _entries;
             int collisionCount = 0;
             if (buckets != null)
             {
-                uint hashCode = (uint)(_comparer?.GetHashCode(key) ?? key.GetHashCode());
+                Debug.Assert(entries != null, "entries should be non-null");
+                uint hashCode = (uint)(_comparer?.GetHashCode(key) ?? key!.GetHashCode()); // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/26761
                 uint bucket = hashCode % (uint)buckets.Length;
                 int last = -1;
                 // Value in buckets is 1-based
@@ -796,11 +804,11 @@ public bool Remove(TKey key)
 
                         if (RuntimeHelpers.IsReferenceOrContainsReferences<TKey>())
                         {
-                            entry.key = default;
+                            entry.key = default!; // TODO-NULLABLE-GENERIC
                         }
                         if (RuntimeHelpers.IsReferenceOrContainsReferences<TValue>())
                         {
-                            entry.value = default;
+                            entry.value = default!; // TODO-NULLABLE-GENERIC
                         }
                         _freeList = i;
                         _freeCount++;
@@ -824,19 +832,20 @@ public bool Remove(TKey key)
         // This overload is a copy of the overload Remove(TKey key) with one additional
         // statement to copy the value for entry being removed into the output parameter.
         // Code has been intentionally duplicated for performance reasons.
-        public bool Remove(TKey key, out TValue value)
+        public bool Remove(TKey key, out TValue value) // TODO-NULLABLE-GENERIC: https://github.com/dotnet/roslyn/issues/26761
         {
             if (key == null)
             {
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.key);
             }
 
-            int[] buckets = _buckets;
-            Entry[] entries = _entries;
+            int[]? buckets = _buckets;
+            Entry[]? entries = _entries;
             int collisionCount = 0;
             if (buckets != null)
             {
-                uint hashCode = (uint)(_comparer?.GetHashCode(key) ?? key.GetHashCode());
+                Debug.Assert(entries != null, "entries should be non-null");
+                uint hashCode = (uint)(_comparer?.GetHashCode(key) ?? key!.GetHashCode()); // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/26761
                 uint bucket = hashCode % (uint)buckets.Length;
                 int last = -1;
                 // Value in buckets is 1-based
@@ -865,11 +874,11 @@ public bool Remove(TKey key, out TValue value)
 
                         if (RuntimeHelpers.IsReferenceOrContainsReferences<TKey>())
                         {
-                            entry.key = default;
+                            entry.key = default!; // TODO-NULLABLE-GENERIC
                         }
                         if (RuntimeHelpers.IsReferenceOrContainsReferences<TValue>())
                         {
-                            entry.value = default;
+                            entry.value = default!; // TODO-NULLABLE-GENERIC
                         }
                         _freeList = i;
                         _freeCount++;
@@ -887,19 +896,19 @@ public bool Remove(TKey key, out TValue value)
                     collisionCount++;
                 }
             }
-            value = default;
+            value = default!; // TODO-NULLABLE-GENERIC
             return false;
         }
 
-        public bool TryGetValue(TKey key, out TValue value)
+        public bool TryGetValue(TKey key, out TValue value) // TODO-NULLABLE-GENERIC: https://github.com/dotnet/roslyn/issues/26761
         {
             int i = FindEntry(key);
             if (i >= 0)
             {
-                value = _entries[i].value;
+                value = _entries![i].value;
                 return true;
             }
-            value = default;
+            value = default!; // TODO-NULLABLE-GENERIC
             return false;
         }
 
@@ -915,7 +924,7 @@ void ICollection.CopyTo(Array array, int index)
         {
             if (array == null)
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
-            if (array.Rank != 1)
+            if (array!.Rank != 1) // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/538
                 ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_RankMultiDimNotSupported);
             if (array.GetLowerBound(0) != 0)
                 ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_NonZeroLowerBound);
@@ -930,10 +939,10 @@ void ICollection.CopyTo(Array array, int index)
             }
             else if (array is DictionaryEntry[] dictEntryArray)
             {
-                Entry[] entries = _entries;
+                Entry[]? entries = _entries;
                 for (int i = 0; i < _count; i++)
                 {
-                    if (entries[i].next >= -1)
+                    if (entries![i].next >= -1)
                     {
                         dictEntryArray[index++] = new DictionaryEntry(entries[i].key, entries[i].value);
                     }
@@ -941,7 +950,7 @@ void ICollection.CopyTo(Array array, int index)
             }
             else
             {
-                object[] objects = array as object[];
+                object[]? objects = array as object[];
                 if (objects == null)
                 {
                     ThrowHelper.ThrowArgumentException_Argument_InvalidArrayType();
@@ -950,12 +959,12 @@ void ICollection.CopyTo(Array array, int index)
                 try
                 {
                     int count = _count;
-                    Entry[] entries = _entries;
+                    Entry[]? entries = _entries;
                     for (int i = 0; i < count; i++)
                     {
-                        if (entries[i].next >= -1)
+                        if (entries![i].next >= -1)
                         {
-                            objects[index++] = new KeyValuePair<TKey, TValue>(entries[i].key, entries[i].value);
+                            objects![index++] = new KeyValuePair<TKey, TValue>(entries[i].key, entries[i].value); // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/26761
                         }
                     }
                 }
@@ -1013,7 +1022,7 @@ public void TrimExcess(int capacity)
                 ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.capacity);
             int newSize = HashHelpers.GetPrime(capacity);
 
-            Entry[] oldEntries = _entries;
+            Entry[]? oldEntries = _entries;
             int currentCapacity = oldEntries == null ? 0 : oldEntries.Length;
             if (newSize >= currentCapacity)
                 return;
@@ -1021,19 +1030,19 @@ public void TrimExcess(int capacity)
             int oldCount = _count;
             _version++;
             Initialize(newSize);
-            Entry[] entries = _entries;
-            int[] buckets = _buckets;
+            Entry[]? entries = _entries;
+            int[]? buckets = _buckets;
             int count = 0;
             for (int i = 0; i < oldCount; i++)
             {
-                uint hashCode = oldEntries[i].hashCode;
+                uint hashCode = oldEntries![i].hashCode; // At this point, we know we have entries.
                 if (oldEntries[i].next >= -1)
                 {
-                    ref Entry entry = ref entries[count];
+                    ref Entry entry = ref entries![count];
                     entry = oldEntries[i];
                     uint bucket = hashCode % (uint)newSize;
                     // Value in _buckets is 1-based
-                    entry.next = buckets[bucket] - 1;
+                    entry.next = buckets![bucket] - 1; // If we get here, we have entries, therefore buckets is not null.
                     // Value in _buckets is 1-based
                     buckets[bucket] = count + 1;
                     count++;
@@ -1055,7 +1064,7 @@ public void TrimExcess(int capacity)
 
         ICollection IDictionary.Values => (ICollection)Values;
 
-        object IDictionary.this[object key]
+        object? IDictionary.this[object key] // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/2384
         {
             get
             {
@@ -1064,7 +1073,7 @@ object IDictionary.this[object key]
                     int i = FindEntry((TKey)key);
                     if (i >= 0)
                     {
-                        return _entries[i].value;
+                        return _entries![i].value;
                     }
                 }
                 return null;
@@ -1079,10 +1088,10 @@ object IDictionary.this[object key]
 
                 try
                 {
-                    TKey tempKey = (TKey)key;
+                    TKey tempKey = (TKey)key!; // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/538
                     try
                     {
-                        this[tempKey] = (TValue)value;
+                        this[tempKey] = (TValue)value!;
                     }
                     catch (InvalidCastException)
                     {
@@ -1105,7 +1114,7 @@ private static bool IsCompatibleKey(object key)
             return (key is TKey);
         }
 
-        void IDictionary.Add(object key, object value)
+        void IDictionary.Add(object key, object? value)
         {
             if (key == null)
             {
@@ -1115,11 +1124,11 @@ void IDictionary.Add(object key, object value)
 
             try
             {
-                TKey tempKey = (TKey)key;
+                TKey tempKey = (TKey)key!; // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/26761
 
                 try
                 {
-                    Add(tempKey, (TValue)value);
+                    Add(tempKey, (TValue)value!);
                 }
                 catch (InvalidCastException)
                 {
@@ -1185,7 +1194,7 @@ public bool MoveNext()
                 // dictionary.count+1 could be negative if dictionary.count is int.MaxValue
                 while ((uint)_index < (uint)_dictionary._count)
                 {
-                    ref Entry entry = ref _dictionary._entries[_index++];
+                    ref Entry entry = ref _dictionary._entries![_index++];
 
                     if (entry.next >= -1)
                     {
@@ -1205,7 +1214,7 @@ public void Dispose()
             {
             }
 
-            object IEnumerator.Current
+            object? IEnumerator.Current // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/23268
             {
                 get
                 {
@@ -1262,7 +1271,7 @@ object IDictionaryEnumerator.Key
                 }
             }
 
-            object IDictionaryEnumerator.Value
+            object? IDictionaryEnumerator.Value
             {
                 get
                 {
@@ -1288,7 +1297,7 @@ public KeyCollection(Dictionary<TKey, TValue> dictionary)
                 {
                     ThrowHelper.ThrowArgumentNullException(ExceptionArgument.dictionary);
                 }
-                _dictionary = dictionary;
+                _dictionary = dictionary!; // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/26761
             }
 
             public Enumerator GetEnumerator()
@@ -1301,21 +1310,21 @@ public void CopyTo(TKey[] array, int index)
                     ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
                 }
 
-                if (index < 0 || index > array.Length)
+                if (index < 0 || index > array!.Length) // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/26761
                 {
                     ThrowHelper.ThrowIndexArgumentOutOfRange_NeedNonNegNumException();
                 }
 
-                if (array.Length - index < _dictionary.Count)
+                if (array!.Length - index < _dictionary.Count) // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/26761
                 {
                     ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_ArrayPlusOffTooSmall);
                 }
 
                 int count = _dictionary._count;
-                Entry[] entries = _dictionary._entries;
+                Entry[]? entries = _dictionary._entries;
                 for (int i = 0; i < count; i++)
                 {
-                    if (entries[i].next >= -1) array[index++] = entries[i].key;
+                    if (entries![i].next >= -1) array[index++] = entries[i].key;
                 }
             }
 
@@ -1348,7 +1357,7 @@ void ICollection.CopyTo(Array array, int index)
             {
                 if (array == null)
                     ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
-                if (array.Rank != 1)
+                if (array!.Rank != 1) // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/26761
                     ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_RankMultiDimNotSupported);
                 if (array.GetLowerBound(0) != 0)
                     ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_NonZeroLowerBound);
@@ -1363,19 +1372,19 @@ void ICollection.CopyTo(Array array, int index)
                 }
                 else
                 {
-                    object[] objects = array as object[];
+                    object[]? objects = array as object[];
                     if (objects == null)
                     {
                         ThrowHelper.ThrowArgumentException_Argument_InvalidArrayType();
                     }
 
                     int count = _dictionary._count;
-                    Entry[] entries = _dictionary._entries;
+                    Entry[]? entries = _dictionary._entries;
                     try
                     {
                         for (int i = 0; i < count; i++)
                         {
-                            if (entries[i].next >= -1) objects[index++] = entries[i].key;
+                            if (entries![i].next >= -1) objects![index++] = entries[i].key; // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/538 (objects)
                         }
                     }
                     catch (ArrayTypeMismatchException)
@@ -1401,7 +1410,7 @@ internal Enumerator(Dictionary<TKey, TValue> dictionary)
                     _dictionary = dictionary;
                     _version = dictionary._version;
                     _index = 0;
-                    _currentKey = default;
+                    _currentKey = default!; // TODO-NULLABLE-GENERIC
                 }
 
                 public void Dispose()
@@ -1417,7 +1426,7 @@ public bool MoveNext()
 
                     while ((uint)_index < (uint)_dictionary._count)
                     {
-                        ref Entry entry = ref _dictionary._entries[_index++];
+                        ref Entry entry = ref _dictionary._entries![_index++];
 
                         if (entry.next >= -1)
                         {
@@ -1427,13 +1436,13 @@ public bool MoveNext()
                     }
 
                     _index = _dictionary._count + 1;
-                    _currentKey = default;
+                    _currentKey = default!; // TODO-NULLABLE-GENERIC
                     return false;
                 }
 
                 public TKey Current => _currentKey;
 
-                object IEnumerator.Current
+                object? IEnumerator.Current // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/23268
                 {
                     get
                     {
@@ -1454,7 +1463,7 @@ void IEnumerator.Reset()
                     }
 
                     _index = 0;
-                    _currentKey = default;
+                    _currentKey = default!; // TODO-NULLABLE-GENERIC
                 }
             }
         }
@@ -1471,7 +1480,7 @@ public ValueCollection(Dictionary<TKey, TValue> dictionary)
                 {
                     ThrowHelper.ThrowArgumentNullException(ExceptionArgument.dictionary);
                 }
-                _dictionary = dictionary;
+                _dictionary = dictionary!; // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/538
             }
 
             public Enumerator GetEnumerator()
@@ -1484,7 +1493,7 @@ public void CopyTo(TValue[] array, int index)
                     ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
                 }
 
-                if (index < 0 || index > array.Length)
+                if ((uint)index > array!.Length) // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/538
                 {
                     ThrowHelper.ThrowIndexArgumentOutOfRange_NeedNonNegNumException();
                 }
@@ -1495,10 +1504,10 @@ public void CopyTo(TValue[] array, int index)
                 }
 
                 int count = _dictionary._count;
-                Entry[] entries = _dictionary._entries;
+                Entry[]? entries = _dictionary._entries;
                 for (int i = 0; i < count; i++)
                 {
-                    if (entries[i].next >= -1) array[index++] = entries[i].value;
+                    if (entries![i].next >= -1) array[index++] = entries[i].value;
                 }
             }
 
@@ -1531,7 +1540,7 @@ void ICollection.CopyTo(Array array, int index)
             {
                 if (array == null)
                     ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
-                if (array.Rank != 1)
+                if (array!.Rank != 1) // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/538
                     ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_RankMultiDimNotSupported);
                 if (array.GetLowerBound(0) != 0)
                     ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_NonZeroLowerBound);
@@ -1546,19 +1555,19 @@ void ICollection.CopyTo(Array array, int index)
                 }
                 else
                 {
-                    object[] objects = array as object[];
+                    object[]? objects = array as object[];
                     if (objects == null)
                     {
                         ThrowHelper.ThrowArgumentException_Argument_InvalidArrayType();
                     }
 
                     int count = _dictionary._count;
-                    Entry[] entries = _dictionary._entries;
+                    Entry[]? entries = _dictionary._entries;
                     try
                     {
                         for (int i = 0; i < count; i++)
                         {
-                            if (entries[i].next >= -1) objects[index++] = entries[i].value;
+                            if (entries![i].next >= -1) objects![index++] = entries[i].value!; // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/538
                         }
                     }
                     catch (ArrayTypeMismatchException)
@@ -1584,7 +1593,7 @@ internal Enumerator(Dictionary<TKey, TValue> dictionary)
                     _dictionary = dictionary;
                     _version = dictionary._version;
                     _index = 0;
-                    _currentValue = default;
+                    _currentValue = default!; // TODO-NULLABLE-GENERIC
                 }
 
                 public void Dispose()
@@ -1600,7 +1609,7 @@ public bool MoveNext()
 
                     while ((uint)_index < (uint)_dictionary._count)
                     {
-                        ref Entry entry = ref _dictionary._entries[_index++];
+                        ref Entry entry = ref _dictionary._entries![_index++];
 
                         if (entry.next >= -1)
                         {
@@ -1609,13 +1618,13 @@ public bool MoveNext()
                         }
                     }
                     _index = _dictionary._count + 1;
-                    _currentValue = default;
+                    _currentValue = default!; // TODO-NULLABLE-GENERIC
                     return false;
                 }
 
                 public TValue Current => _currentValue;
 
-                object IEnumerator.Current
+                object? IEnumerator.Current
                 {
                     get
                     {
@@ -1635,7 +1644,7 @@ void IEnumerator.Reset()
                         ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumFailedVersion();
                     }
                     _index = 0;
-                    _currentValue = default;
+                    _currentValue = default!; // TODO-NULLABLE-GENERIC
                 }
             }
         }
diff --git a/src/Common/src/CoreLib/System/Collections/Generic/EqualityComparer.cs b/src/Common/src/CoreLib/System/Collections/Generic/EqualityComparer.cs
index 01204775e926..fdaf3bc858ee 100644
--- a/src/Common/src/CoreLib/System/Collections/Generic/EqualityComparer.cs
+++ b/src/Common/src/CoreLib/System/Collections/Generic/EqualityComparer.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;
 using System.Runtime.Serialization;
 
@@ -14,9 +15,11 @@ public abstract partial class EqualityComparer<T> : IEqualityComparer, IEquality
         // public static EqualityComparer<T> Default is runtime-specific
 
         public abstract bool Equals(T x, T y);
-        public abstract int GetHashCode(T obj);
+        public abstract int GetHashCode(T obj); // TODO-NULLABLE-GENERIC: Shouldn't accept nulls.
 
-        int IEqualityComparer.GetHashCode(object obj)
+#pragma warning disable CS8617 // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/30958
+        int IEqualityComparer.GetHashCode(object? obj)
+#pragma warning restore CS8617
         {
             if (obj == null) return 0;
             if (obj is T) return GetHashCode((T)obj);
@@ -24,7 +27,7 @@ int IEqualityComparer.GetHashCode(object obj)
             return 0;
         }
 
-        bool IEqualityComparer.Equals(object x, object y)
+        bool IEqualityComparer.Equals(object? x, object? y)
         {
             if (x == y) return true;
             if (x == null || y == null) return false;
@@ -58,7 +61,7 @@ public override bool Equals(T x, T y)
 
         // Equals method for the comparer itself.
         // If in the future this type is made sealed, change the is check to obj != null && GetType() == obj.GetType().
-        public override bool Equals(object obj) =>
+        public override bool Equals(object? obj) =>
             obj is GenericEqualityComparer<T>;
 
         // If in the future this type is made sealed, change typeof(...) to GetType().
@@ -87,7 +90,7 @@ public override bool Equals(T? x, T? y)
         public override int GetHashCode(T? obj) => obj.GetHashCode();
 
         // Equals method for the comparer itself.
-        public override bool Equals(object obj) =>
+        public override bool Equals(object? obj) =>
             obj != null && GetType() == obj.GetType();
 
         public override int GetHashCode() =>
@@ -115,7 +118,7 @@ public override bool Equals(T x, T y)
         public override int GetHashCode(T obj) => obj?.GetHashCode() ?? 0;
 
         // Equals method for the comparer itself.
-        public override bool Equals(object obj) =>
+        public override bool Equals(object? obj) =>
             obj != null && GetType() == obj.GetType();
 
         public override int GetHashCode() =>
@@ -140,7 +143,7 @@ public override int GetHashCode(byte b)
         }
 
         // Equals method for the comparer itself.
-        public override bool Equals(object obj) =>
+        public override bool Equals(object? obj) =>
             obj != null && GetType() == obj.GetType();
 
         public override int GetHashCode() =>
@@ -174,7 +177,7 @@ public override int GetHashCode(T obj)
         }
 
         // Equals method for the comparer itself.
-        public override bool Equals(object obj) =>
+        public override bool Equals(object? obj) =>
             obj != null && GetType() == obj.GetType();
 
         public override int GetHashCode() =>
diff --git a/src/Common/src/CoreLib/System/Collections/Generic/KeyValuePair.cs b/src/Common/src/CoreLib/System/Collections/Generic/KeyValuePair.cs
index 82c786d407c3..18b3863e7856 100644
--- a/src/Common/src/CoreLib/System/Collections/Generic/KeyValuePair.cs
+++ b/src/Common/src/CoreLib/System/Collections/Generic/KeyValuePair.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.ComponentModel;
 using System.Text;
 
@@ -19,7 +20,7 @@ public static KeyValuePair<TKey, TValue> Create<TKey, TValue>(TKey key, TValue v
         /// <summary>
         /// Used by KeyValuePair.ToString to reduce generic code
         /// </summary>
-        internal static string PairToString(object key, object value)
+        internal static string PairToString(object? key, object? value)
         {
             StringBuilder s = StringBuilderCache.Acquire();
             s.Append('[');
diff --git a/src/Common/src/CoreLib/System/Collections/IDictionary.cs b/src/Common/src/CoreLib/System/Collections/IDictionary.cs
index b077c9192754..3ecf66992985 100644
--- a/src/Common/src/CoreLib/System/Collections/IDictionary.cs
+++ b/src/Common/src/CoreLib/System/Collections/IDictionary.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
 
 namespace System.Collections
 {
@@ -15,7 +15,7 @@ public interface IDictionary : ICollection
         // Interfaces are not serializable
         // The Item property provides methods to read and edit entries 
         // in the Dictionary.
-        object this[object key]
+        object? this[object key] // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/2384
         {
             get;
             set;
@@ -39,7 +39,7 @@ ICollection Values
 
         // Adds a key-value pair to the dictionary.
         // 
-        void Add(object key, object value);
+        void Add(object key, object? value);
 
         // Removes all pairs from the dictionary.
         void Clear();
diff --git a/src/Common/src/CoreLib/System/Collections/IDictionaryEnumerator.cs b/src/Common/src/CoreLib/System/Collections/IDictionaryEnumerator.cs
index 0cf6aaa1545d..59b6f7839a6f 100644
--- a/src/Common/src/CoreLib/System/Collections/IDictionaryEnumerator.cs
+++ b/src/Common/src/CoreLib/System/Collections/IDictionaryEnumerator.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.Collections
 {
     // This interface represents an enumerator that allows sequential access to the
@@ -50,7 +51,7 @@ object Key
         // to GetValue with no intervening calls to GetNext will
         // return the same object.
         // 
-        object Value
+        object? Value
         {
             get;
         }
diff --git a/src/Common/src/CoreLib/System/Collections/IEqualityComparer.cs b/src/Common/src/CoreLib/System/Collections/IEqualityComparer.cs
index 9b5476896ce6..c2c290c44b4e 100644
--- a/src/Common/src/CoreLib/System/Collections/IEqualityComparer.cs
+++ b/src/Common/src/CoreLib/System/Collections/IEqualityComparer.cs
@@ -2,15 +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.
 
-using System;
-
+#nullable enable
 namespace System.Collections
 {
     // An IEqualityComparer is a mechanism to consume custom performant comparison infrastructure
     // that can be consumed by some of the common collections.
     public interface IEqualityComparer
     {
-        bool Equals(object x, object y);
+        bool Equals(object? x, object? y);
         int GetHashCode(object obj);
     }
 }
diff --git a/src/Common/src/CoreLib/System/Security/SecurityElement.cs b/src/Common/src/CoreLib/System/Security/SecurityElement.cs
index 05c13c8a02af..197eaa079b0c 100644
--- a/src/Common/src/CoreLib/System/Security/SecurityElement.cs
+++ b/src/Common/src/CoreLib/System/Security/SecurityElement.cs
@@ -145,7 +145,7 @@ public Hashtable? Attributes
                     while (enumerator.MoveNext())
                     {
                         string attrName = (string)enumerator.Key;
-                        string attrValue = (string)enumerator.Value;
+                        string? attrValue = (string?)enumerator.Value;
 
                         if (!IsValidAttributeName(attrName))
                             throw new ArgumentException(SR.Format(SR.Argument_InvalidElementName, attrName));
diff --git a/src/Common/src/CoreLib/System/Tuple.cs b/src/Common/src/CoreLib/System/Tuple.cs
index cf996005129d..bcfd4ce11572 100644
--- a/src/Common/src/CoreLib/System/Tuple.cs
+++ b/src/Common/src/CoreLib/System/Tuple.cs
@@ -157,7 +157,7 @@ public override int GetHashCode()
 
         int IStructuralEquatable.GetHashCode(IEqualityComparer comparer)
         {
-            return comparer.GetHashCode(m_Item1);
+            return comparer.GetHashCode(m_Item1!); // TODO-NULLABLE-GENERIC
         }
 
         int ITupleInternal.GetHashCode(IEqualityComparer comparer)
@@ -262,7 +262,7 @@ public override int GetHashCode()
 
         int IStructuralEquatable.GetHashCode(IEqualityComparer comparer)
         {
-            return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item1), comparer.GetHashCode(m_Item2));
+            return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item1!), comparer.GetHashCode(m_Item2!)); // TODO-NULLABLE-GENERIC
         }
 
         int ITupleInternal.GetHashCode(IEqualityComparer comparer)
@@ -380,7 +380,7 @@ public override int GetHashCode()
 
         int IStructuralEquatable.GetHashCode(IEqualityComparer comparer)
         {
-            return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item1), comparer.GetHashCode(m_Item2), comparer.GetHashCode(m_Item3));
+            return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item1!), comparer.GetHashCode(m_Item2!), comparer.GetHashCode(m_Item3!)); // TODO-NULLABLE-GENERIC
         }
 
         int ITupleInternal.GetHashCode(IEqualityComparer comparer)
@@ -509,7 +509,7 @@ public override int GetHashCode()
 
         int IStructuralEquatable.GetHashCode(IEqualityComparer comparer)
         {
-            return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item1), comparer.GetHashCode(m_Item2), comparer.GetHashCode(m_Item3), comparer.GetHashCode(m_Item4));
+            return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item1!), comparer.GetHashCode(m_Item2!), comparer.GetHashCode(m_Item3!), comparer.GetHashCode(m_Item4!)); // TODO-NULLABLE-GENERIC
         }
 
         int ITupleInternal.GetHashCode(IEqualityComparer comparer)
@@ -649,7 +649,7 @@ public override int GetHashCode()
 
         int IStructuralEquatable.GetHashCode(IEqualityComparer comparer)
         {
-            return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item1), comparer.GetHashCode(m_Item2), comparer.GetHashCode(m_Item3), comparer.GetHashCode(m_Item4), comparer.GetHashCode(m_Item5));
+            return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item1!), comparer.GetHashCode(m_Item2!), comparer.GetHashCode(m_Item3!), comparer.GetHashCode(m_Item4!), comparer.GetHashCode(m_Item5!)); // TODO-NULLABLE-GENERIC
         }
 
         int ITupleInternal.GetHashCode(IEqualityComparer comparer)
@@ -800,7 +800,7 @@ public override int GetHashCode()
 
         int IStructuralEquatable.GetHashCode(IEqualityComparer comparer)
         {
-            return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item1), comparer.GetHashCode(m_Item2), comparer.GetHashCode(m_Item3), comparer.GetHashCode(m_Item4), comparer.GetHashCode(m_Item5), comparer.GetHashCode(m_Item6));
+            return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item1!), comparer.GetHashCode(m_Item2!), comparer.GetHashCode(m_Item3!), comparer.GetHashCode(m_Item4!), comparer.GetHashCode(m_Item5!), comparer.GetHashCode(m_Item6!)); // TODO-NULLABLE-GENERIC
         }
 
         int ITupleInternal.GetHashCode(IEqualityComparer comparer)
@@ -962,7 +962,7 @@ public override int GetHashCode()
 
         int IStructuralEquatable.GetHashCode(IEqualityComparer comparer)
         {
-            return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item1), comparer.GetHashCode(m_Item2), comparer.GetHashCode(m_Item3), comparer.GetHashCode(m_Item4), comparer.GetHashCode(m_Item5), comparer.GetHashCode(m_Item6), comparer.GetHashCode(m_Item7));
+            return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item1!), comparer.GetHashCode(m_Item2!), comparer.GetHashCode(m_Item3!), comparer.GetHashCode(m_Item4!), comparer.GetHashCode(m_Item5!), comparer.GetHashCode(m_Item6!), comparer.GetHashCode(m_Item7!)); // TODO-NULLABLE-GENERIC
         }
 
         int ITupleInternal.GetHashCode(IEqualityComparer comparer)
@@ -1149,19 +1149,19 @@ int IStructuralEquatable.GetHashCode(IEqualityComparer comparer)
             switch (k)
             {
                 case 1:
-                    return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item7), t.GetHashCode(comparer));
+                    return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item7!), t.GetHashCode(comparer)); // TODO-NULLABLE-GENERIC
                 case 2:
-                    return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item6), comparer.GetHashCode(m_Item7), t.GetHashCode(comparer));
+                    return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item6!), comparer.GetHashCode(m_Item7!), t.GetHashCode(comparer)); // TODO-NULLABLE-GENERIC
                 case 3:
-                    return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item5), comparer.GetHashCode(m_Item6), comparer.GetHashCode(m_Item7), t.GetHashCode(comparer));
+                    return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item5!), comparer.GetHashCode(m_Item6!), comparer.GetHashCode(m_Item7!), t.GetHashCode(comparer)); // TODO-NULLABLE-GENERIC
                 case 4:
-                    return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item4), comparer.GetHashCode(m_Item5), comparer.GetHashCode(m_Item6), comparer.GetHashCode(m_Item7), t.GetHashCode(comparer));
+                    return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item4!), comparer.GetHashCode(m_Item5!), comparer.GetHashCode(m_Item6!), comparer.GetHashCode(m_Item7!), t.GetHashCode(comparer)); // TODO-NULLABLE-GENERIC
                 case 5:
-                    return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item3), comparer.GetHashCode(m_Item4), comparer.GetHashCode(m_Item5), comparer.GetHashCode(m_Item6), comparer.GetHashCode(m_Item7), t.GetHashCode(comparer));
+                    return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item3!), comparer.GetHashCode(m_Item4!), comparer.GetHashCode(m_Item5!), comparer.GetHashCode(m_Item6!), comparer.GetHashCode(m_Item7!), t.GetHashCode(comparer)); // TODO-NULLABLE-GENERIC
                 case 6:
-                    return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item2), comparer.GetHashCode(m_Item3), comparer.GetHashCode(m_Item4), comparer.GetHashCode(m_Item5), comparer.GetHashCode(m_Item6), comparer.GetHashCode(m_Item7), t.GetHashCode(comparer));
+                    return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item2!), comparer.GetHashCode(m_Item3!), comparer.GetHashCode(m_Item4!), comparer.GetHashCode(m_Item5!), comparer.GetHashCode(m_Item6!), comparer.GetHashCode(m_Item7!), t.GetHashCode(comparer)); // TODO-NULLABLE-GENERIC
                 case 7:
-                    return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item1), comparer.GetHashCode(m_Item2), comparer.GetHashCode(m_Item3), comparer.GetHashCode(m_Item4), comparer.GetHashCode(m_Item5), comparer.GetHashCode(m_Item6), comparer.GetHashCode(m_Item7), t.GetHashCode(comparer));
+                    return Tuple.CombineHashCodes(comparer.GetHashCode(m_Item1!), comparer.GetHashCode(m_Item2!), comparer.GetHashCode(m_Item3!), comparer.GetHashCode(m_Item4!), comparer.GetHashCode(m_Item5!), comparer.GetHashCode(m_Item6!), comparer.GetHashCode(m_Item7!), t.GetHashCode(comparer)); // TODO-NULLABLE-GENERIC
             }
             Debug.Fail("Missed all cases for computing Tuple hash code");
             return -1;
diff --git a/src/Common/src/CoreLib/System/ValueTuple.cs b/src/Common/src/CoreLib/System/ValueTuple.cs
index 0fd476e00ff6..b227535d2762 100644
--- a/src/Common/src/CoreLib/System/ValueTuple.cs
+++ b/src/Common/src/CoreLib/System/ValueTuple.cs
@@ -412,12 +412,12 @@ public override int GetHashCode()
 
         int IStructuralEquatable.GetHashCode(IEqualityComparer comparer)
         {
-            return comparer.GetHashCode(Item1);
+            return comparer.GetHashCode(Item1!); // TODO-NULLABLE-GENERIC
         }
 
         int IValueTupleInternal.GetHashCode(IEqualityComparer comparer)
         {
-            return comparer.GetHashCode(Item1);
+            return comparer.GetHashCode(Item1!); // TODO-NULLABLE-GENERIC
         }
 
         /// <summary>
@@ -616,8 +616,8 @@ int IStructuralEquatable.GetHashCode(IEqualityComparer comparer)
 
         private int GetHashCodeCore(IEqualityComparer comparer)
         {
-            return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item1),
-                                               comparer.GetHashCode(Item2));
+            return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item1!),
+                                               comparer.GetHashCode(Item2!)); // TODO-NULLABLE-GENERIC
         }
 
         int IValueTupleInternal.GetHashCode(IEqualityComparer comparer)
@@ -823,9 +823,9 @@ int IStructuralEquatable.GetHashCode(IEqualityComparer comparer)
 
         private int GetHashCodeCore(IEqualityComparer comparer)
         {
-            return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item1),
-                                               comparer.GetHashCode(Item2),
-                                               comparer.GetHashCode(Item3));
+            return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item1!),
+                                               comparer.GetHashCode(Item2!),
+                                               comparer.GetHashCode(Item3!)); // TODO-NULLABLE-GENERIC
         }
 
         int IValueTupleInternal.GetHashCode(IEqualityComparer comparer)
@@ -1047,10 +1047,10 @@ int IStructuralEquatable.GetHashCode(IEqualityComparer comparer)
 
         private int GetHashCodeCore(IEqualityComparer comparer)
         {
-            return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item1),
-                                               comparer.GetHashCode(Item2),
-                                               comparer.GetHashCode(Item3),
-                                               comparer.GetHashCode(Item4));
+            return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item1!),
+                                               comparer.GetHashCode(Item2!),
+                                               comparer.GetHashCode(Item3!),
+                                               comparer.GetHashCode(Item4!)); // TODO-NULLABLE-GENERIC
         }
 
         int IValueTupleInternal.GetHashCode(IEqualityComparer comparer)
@@ -1290,11 +1290,11 @@ int IStructuralEquatable.GetHashCode(IEqualityComparer comparer)
 
         private int GetHashCodeCore(IEqualityComparer comparer)
         {
-            return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item1),
-                                               comparer.GetHashCode(Item2),
-                                               comparer.GetHashCode(Item3),
-                                               comparer.GetHashCode(Item4),
-                                               comparer.GetHashCode(Item5));
+            return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item1!),
+                                               comparer.GetHashCode(Item2!),
+                                               comparer.GetHashCode(Item3!),
+                                               comparer.GetHashCode(Item4!),
+                                               comparer.GetHashCode(Item5!)); // TODO-NULLABLE-GENERIC
         }
 
         int IValueTupleInternal.GetHashCode(IEqualityComparer comparer)
@@ -1552,12 +1552,12 @@ int IStructuralEquatable.GetHashCode(IEqualityComparer comparer)
 
         private int GetHashCodeCore(IEqualityComparer comparer)
         {
-            return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item1),
-                                               comparer.GetHashCode(Item2),
-                                               comparer.GetHashCode(Item3),
-                                               comparer.GetHashCode(Item4),
-                                               comparer.GetHashCode(Item5),
-                                               comparer.GetHashCode(Item6));
+            return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item1!),
+                                               comparer.GetHashCode(Item2!),
+                                               comparer.GetHashCode(Item3!),
+                                               comparer.GetHashCode(Item4!),
+                                               comparer.GetHashCode(Item5!),
+                                               comparer.GetHashCode(Item6!)); // TODO-NULLABLE-GENERIC
         }
 
         int IValueTupleInternal.GetHashCode(IEqualityComparer comparer)
@@ -1833,13 +1833,13 @@ int IStructuralEquatable.GetHashCode(IEqualityComparer comparer)
 
         private int GetHashCodeCore(IEqualityComparer comparer)
         {
-            return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item1),
-                                               comparer.GetHashCode(Item2),
-                                               comparer.GetHashCode(Item3),
-                                               comparer.GetHashCode(Item4),
-                                               comparer.GetHashCode(Item5),
-                                               comparer.GetHashCode(Item6),
-                                               comparer.GetHashCode(Item7));
+            return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item1!),
+                                               comparer.GetHashCode(Item2!),
+                                               comparer.GetHashCode(Item3!),
+                                               comparer.GetHashCode(Item4!),
+                                               comparer.GetHashCode(Item5!),
+                                               comparer.GetHashCode(Item6!),
+                                               comparer.GetHashCode(Item7!)); // TODO-NULLABLE-GENERIC
         }
 
         int IValueTupleInternal.GetHashCode(IEqualityComparer comparer)
@@ -2202,9 +2202,9 @@ private int GetHashCodeCore(IEqualityComparer comparer)
             IValueTupleInternal? rest = Rest as IValueTupleInternal;
             if (rest == null)
             {
-                return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item1), comparer.GetHashCode(Item2), comparer.GetHashCode(Item3),
-                                                   comparer.GetHashCode(Item4), comparer.GetHashCode(Item5), comparer.GetHashCode(Item6),
-                                                   comparer.GetHashCode(Item7));
+                return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item1!), comparer.GetHashCode(Item2!), comparer.GetHashCode(Item3!),
+                                                   comparer.GetHashCode(Item4!), comparer.GetHashCode(Item5!), comparer.GetHashCode(Item6!),
+                                                   comparer.GetHashCode(Item7!)); // TODO-NULLABLE-GENERIC
             }
 
             int size = rest.Length;
@@ -2215,27 +2215,27 @@ private int GetHashCodeCore(IEqualityComparer comparer)
             switch (k)
             {
                 case 1:
-                    return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item7), rest.GetHashCode(comparer));
+                    return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item7!), rest.GetHashCode(comparer)); // TODO-NULLABLE-GENERIC
                 case 2:
-                    return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item6), comparer.GetHashCode(Item7), rest.GetHashCode(comparer));
+                    return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item6!), comparer.GetHashCode(Item7!), rest.GetHashCode(comparer)); // TODO-NULLABLE-GENERIC
                 case 3:
-                    return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item5), comparer.GetHashCode(Item6), comparer.GetHashCode(Item7),
-                                                       rest.GetHashCode(comparer));
+                    return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item5!), comparer.GetHashCode(Item6!), comparer.GetHashCode(Item7!),
+                                                       rest.GetHashCode(comparer)); // TODO-NULLABLE-GENERIC
                 case 4:
-                    return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item4), comparer.GetHashCode(Item5), comparer.GetHashCode(Item6),
-                                                       comparer.GetHashCode(Item7), rest.GetHashCode(comparer));
+                    return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item4!), comparer.GetHashCode(Item5!), comparer.GetHashCode(Item6!),
+                                                       comparer.GetHashCode(Item7!), rest.GetHashCode(comparer)); // TODO-NULLABLE-GENERIC
                 case 5:
-                    return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item3), comparer.GetHashCode(Item4), comparer.GetHashCode(Item5),
-                                                       comparer.GetHashCode(Item6), comparer.GetHashCode(Item7), rest.GetHashCode(comparer));
+                    return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item3!), comparer.GetHashCode(Item4!), comparer.GetHashCode(Item5!),
+                                                       comparer.GetHashCode(Item6!), comparer.GetHashCode(Item7!), rest.GetHashCode(comparer)); // TODO-NULLABLE-GENERIC
                 case 6:
-                    return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item2), comparer.GetHashCode(Item3), comparer.GetHashCode(Item4),
-                                                       comparer.GetHashCode(Item5), comparer.GetHashCode(Item6), comparer.GetHashCode(Item7),
-                                                       rest.GetHashCode(comparer));
+                    return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item2!), comparer.GetHashCode(Item3!), comparer.GetHashCode(Item4!),
+                                                       comparer.GetHashCode(Item5!), comparer.GetHashCode(Item6!), comparer.GetHashCode(Item7!),
+                                                       rest.GetHashCode(comparer)); // TODO-NULLABLE-GENERIC
                 case 7:
                 case 8:
-                    return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item1), comparer.GetHashCode(Item2), comparer.GetHashCode(Item3),
-                                                       comparer.GetHashCode(Item4), comparer.GetHashCode(Item5), comparer.GetHashCode(Item6),
-                                                       comparer.GetHashCode(Item7), rest.GetHashCode(comparer));
+                    return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item1!), comparer.GetHashCode(Item2!), comparer.GetHashCode(Item3!),
+                                                       comparer.GetHashCode(Item4!), comparer.GetHashCode(Item5!), comparer.GetHashCode(Item6!),
+                                                       comparer.GetHashCode(Item7!), rest.GetHashCode(comparer)); // TODO-NULLABLE-GENERIC
             }
 
             Debug.Fail("Missed all cases for computing ValueTuple hash code");