Make DBNull serializable

src/mscorlib/shared/System.Private.CoreLib.Shared.projitems

@@ -486,6 +486,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\UnauthorizedAccessException.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\UnhandledExceptionEventArgs.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\UnhandledExceptionEventHandler.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\UnitySerializationHolder.cs"/> 
     <Compile Include="$(MSBuildThisFileDirectory)System\ValueTuple.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Version.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Void.cs" />

src/mscorlib/shared/System/DBNull.cs

@@ -6,17 +6,23 @@
 
 namespace System
 {
+    [Serializable]
     public sealed class DBNull : ISerializable, IConvertible
     {
         private DBNull()
         {
         }
-
+
+        private DBNull(SerializationInfo info, StreamingContext context)
+        {
+            throw new NotSupportedException(SR.NotSupported_DBNullSerial);
+        }
+
         public static readonly DBNull Value = new DBNull();
 
         public void GetObjectData(SerializationInfo info, StreamingContext context)
         {
-            throw new PlatformNotSupportedException();
+            UnitySerializationHolder.GetUnitySerializationInfo(info, UnitySerializationHolder.NullUnity, null, null);
         }
 
         public override string ToString()

src/mscorlib/shared/System/UnitySerializationHolder.cs

@@ -0,0 +1,56 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// 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.Runtime.Serialization;
+using System.Reflection;
+
+namespace System
+{
+    /// <summary>
+    /// Holds Null class for which we guarantee that there is only ever one instance of.
+    /// This only exists for backwarts compatibility with 
+    /// </summary>
+#if CORECLR
+    internal
+#else
+    public  // On CoreRT this must be public.
+#endif
+    sealed class UnitySerializationHolder : ISerializable, IObjectReference
+    {
+        internal const int NullUnity = 0x0002;
+
+        public static void GetUnitySerializationInfo(SerializationInfo info, int unityType, string data, Assembly assembly)
+        {
+            // A helper method that returns the SerializationInfo that a class utilizing 
+            // UnitySerializationHelper should return from a call to GetObjectData.  It contains
+            // the unityType (defined above) and any optional data (used only for the reflection
+            // types.)
+
+            info.SetType(typeof(UnitySerializationHolder));
+            info.AddValue("Data", data, typeof(string));
+            info.AddValue("UnityType", unityType);
+            info.AddValue("AssemblyName", assembly?.FullName ?? string.Empty);
+        }
+
+        public UnitySerializationHolder(SerializationInfo info, StreamingContext context)
+        {
+            if (info == null)
+            {
+                throw new ArgumentNullException(nameof(info));
+            }
+
+            // We are ignoring any serialization input as we are only concerned about DBNull.
+        }
+
+        public void GetObjectData(SerializationInfo info, StreamingContext context) =>
+            throw new NotSupportedException(SR.NotSupported_UnitySerHolder);
+
+        public object GetRealObject(StreamingContext context)
+        {
+            // We are always returning the same DBNull instance and ignoring serialization input.
+
+            return DBNull.Value;
+        }
+    }
+}