Nullable: src\System\Runtime\InteropServices (dotnet/coreclr#23936)

* Nullable: src\System\Runtime\InteropServices

* Address PR feedback

Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>

src/System.Private.CoreLib/shared/System/Delegate.cs

@@ -13,15 +13,15 @@ public abstract partial class Delegate : ICloneable, ISerializable
     {
         public virtual object Clone() => MemberwiseClone();
 
-        public static Delegate? Combine(Delegate? a, Delegate? b)
+        public static Delegate? Combine(Delegate? a, Delegate? b) // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/26761
         {
             if (a is null)
                 return b;
 
             return a.CombineImpl(b);
         }
 
-        public static Delegate? Combine(params Delegate?[]? delegates)
+        public static Delegate? Combine(params Delegate?[]? delegates) // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/26761
         {
             if (delegates == null || delegates.Length == 0)
                 return null;

src/System.Private.CoreLib/shared/System/Gen2GcCallback.cs

@@ -39,7 +39,7 @@ public static void Register(Func<object, bool> callback, object targetObj)
         ~Gen2GcCallback()
         {
             // Check to see if the target object is still alive.
-            object targetObj = _weakTargetObj.Target;
+            object? targetObj = _weakTargetObj.Target;
             if (targetObj == null)
             {
                 // The target object is dead, so this callback object is no longer needed.

src/System.Private.CoreLib/shared/System/Runtime/InteropServices/GCHandle.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.Threading;
@@ -34,7 +35,7 @@ public partial struct GCHandle
         private IntPtr _handle;
 
         // Allocate a handle storing the object and the type.
-        private GCHandle(object value, GCHandleType type)
+        private GCHandle(object? value, GCHandleType type)
         {
             // Make sure the type parameter is within the valid range for the enum.
             if ((uint)type > (uint)GCHandleType.Pinned) // IMPORTANT: This must be kept in sync with the GCHandleType enum.
@@ -64,13 +65,13 @@ private GCHandle(object value, GCHandleType type)
         /// <summary>Creates a new GC handle for an object.</summary>
         /// <param name="value">The object that the GC handle is created for.</param>
         /// <returns>A new GC handle that protects the object.</returns>
-        public static GCHandle Alloc(object value) => new GCHandle(value, GCHandleType.Normal);
+        public static GCHandle Alloc(object? value) => new GCHandle(value, GCHandleType.Normal);
 
         /// <summary>Creates a new GC handle for an object.</summary>
         /// <param name="value">The object that the GC handle is created for.</param>
         /// <param name="type">The type of GC handle to create.</param>
         /// <returns>A new GC handle that protects the object.</returns>
-        public static GCHandle Alloc(object value, GCHandleType type) => new GCHandle(value, type);
+        public static GCHandle Alloc(object? value, GCHandleType type) => new GCHandle(value, type);
 
         /// <summary>Frees a GC handle.</summary>
         public void Free()
@@ -82,7 +83,7 @@ public void Free()
         }
 
         // Target property - allows getting / updating of the handle's referent.
-        public object Target
+        public object? Target
         {
             get
             {
@@ -168,7 +169,7 @@ public static GCHandle FromIntPtr(IntPtr value)
 
         public override int GetHashCode() => _handle.GetHashCode();
 
-        public override bool Equals(object o) => o is GCHandle && _handle == ((GCHandle)o)._handle;
+        public override bool Equals(object? o) => o is GCHandle && _handle == ((GCHandle)o)._handle;
 
         public static bool operator ==(GCHandle a, GCHandle b) => a._handle == b._handle;