Need to handle incoming IDataObject pointers for AOT

[JeremyKuhne]

In order to support trimming/AOT we'll be moving our OLE interactions to native pointers and ComWrappers. In doing that we'll need to handle incoming IDataObject pointers. This issue is to capture some of what this entails.

When we get IDataObject* from OLE (for drag/drop and clipboard) we have a number of cases:

1. The IDataObject* is a CCW from COM interop.
2. The IDataObject* is a CCW from ComWrappers
3. The IDataObject* is neither (originally created in native code - that is originating outside of .NET).

In the first case we need to recognize that and unwrap the managed object if it was one that we populated. It is possible someone else may have manually P/Invoked data out and we need to be aware of it and consider if we're breaking any casting support that COM interop provides.

The second case is much the same, we want to get the original DataObject out and not try and wrap in a ComWrappers RCW.

Third case is a bit more tricky. Historically we have wrapped this in a COM interop RCW. This allows dynamic casting of any supported COM interface via QueryInterface. While we can create a ComWrappers RCW, we wouldn't get this behavior anymore in that case.

For example in CreateDragEventArgs:

object? obj = Marshal.GetObjectForIUnknown((nint)pDataObj);
if (obj is IDataObject dataObject)
{
    data = dataObject;
}
else if (obj is ComTypes.IDataObject nativeDataObject)
{
    data = new DataObject(nativeDataObject);
}

Because of this lack of dynamic casting we'll have to either use built in COM when it is available and/or allow configuring the behavior based on config switches.

[elachlan]

Unsure if correct place, apologies if not.

Ole32.ReleaseStgMedium uses System.Runtime.InteropServices.ComTypes. So I can't convert it to CsWin32. Should ReleaseStgMedium be added to the System.Runtime.InteropServices namespace via runtime? My quick google sees quite a few people implementing their own DllImport where it would make sense to just expose it via runtime.

[elachlan]

Also are we going to continue to use the types from System.Runtime.InteropServices.ComTypes or move to the ones generated from cswin32?