ComWrappers feature imbalance between platforms

[b-iotti]

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)

[AaronRobinsonMSFT]

The addition of the new "unwrapping" APIs on `ComWrappers makes the final checkbox above unnecessary. The API disparity still exists but at this point there is no strong push for it be addressed. If people find this issue, we can reconsider on an API by API basis.