Fix CA2256 in COM generator when using 'ManagedObjectWrapper' (#114642)

Author: Sergio0694

src/libraries/System.Runtime.InteropServices/gen/ComInterfaceGenerator/ComInterfaceGenerator.cs

@@ -185,6 +185,18 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
                     using StringWriter source = new();
                     source.WriteLine("// <auto-generated />");
                     source.WriteLine("#pragma warning disable CS0612, CS0618"); // Suppress warnings about [Obsolete] member usage in generated code.
+
+                    // If the user has specified 'ManagedObjectWrapper', it means that the COM interface will never be used to marshal a native
+                    // object as an RCW (eg. the IDIC vtable will also not be generated, nor any additional supporting code). To reduce binary
+                    // size, we're not emitting the interface methods on the implementation interface that has '[DynamicInterfaceCastableImplementation]'
+                    // on it. However, doing so will cause the CA2256 warning to be produced. We can't remove the attribute, as that would cause
+                    // the wrong exception to be thrown when trying an IDIC cast with this interface (not 'InvalidCastException'). Because this is
+                    // a niche scenario, and we don't want to regress perf or size, we can just disable the warning instead.
+                    if (interfaceContext.Options is ComInterfaceOptions.ManagedObjectWrapper)
+                    {
+                        source.WriteLine("#pragma warning disable CA2256");
+                    }
+
                     interfaceInfo.WriteTo(source);
                     // Two newlines looks cleaner than one
                     source.WriteLine();

src/libraries/System.Runtime.InteropServices/src/System/Runtime/InteropServices/Marshalling/ComInterfaceOptions.cs

@@ -23,6 +23,17 @@ public enum ComInterfaceOptions
         /// <summary>
         /// Generate a wrapper for managed objects to enable exposing them through the COM interface.
         /// </summary>
+        /// <remarks>
+        /// <para>
+        /// When this flag is the only one specified on a given COM interface, no implementation methods will be generated
+        /// to support <see cref="System.Runtime.InteropServices.IDynamicInterfaceCastable"/> casts. In such a scenario,
+        /// attempting to cast to that interface type will still succeed, but any calls to interface methods will fail.
+        /// </para>
+        /// <para>
+        /// Using only this flag is purely a binary size optimization. If calling methods via this interface on native
+        /// object is required, the <see cref="ComObjectWrapper"/> flag should also be used instead.
+        /// </para>
+        /// </remarks>
         ManagedObjectWrapper = 0x1,
         /// <summary>
         /// Generate a wrapper for COM objects to enable exposing them through the managed interface.