ComWrappers feature imbalance between platforms #56474

As a part of enabling support for ComWrappers on non-Windows platforms, the feature availability across platforms is now different.

Once the contents of #54838 are in place, non-Windows platforms will be allowed to use the IUnknown interfaces.

Currently, the following are Windows-only:

• Marshalling - Some APIs, such as Marshal.GetIUnknownForObject, respect the "Global registration" of ComWrappers but currently those APIs are Windows only. We should enable all Marshal APIs that support the ComWrappers API.
  • Related: Marshal.ReleaseComObject does not account for ComWrappers #54317
• Update existing API testing Add test coverage for Marshal.AddRef/Release/QueryInterface on non-Windows #51072
• Platform compatibility metadata - ComWrappers is currently marked as only supported on Windows:
  

  runtime/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/ComWrappers.cs

  Lines 72 to 74 in a6b9a63

  	[SupportedOSPlatform("windows")]
  	[CLSCompliant(false)]
  	public abstract partial class ComWrappers

  • Should be able to remove that attribute on the ComWrappers class
  • Might be a good idea to mark just the RegisterForMarshalling method as only supported on windows (based on the lack of marshalling with the global instance above)