Handle wrapping COM pointers in managed objects

[JeremyKuhne]

Built-in COM interop is not available when trimming or AOT are enabled. We need to use ComWrappers in those scenarios. This is important where we currently publicly expose COM RCWs. One concrete example of this is in AxHost. AxHost.GetOcx exposes the ActiveX instance and it is also exposed via the customizable construction virtual AxHost.CreateInstanceCore(Guid clsid).

We need to add a helper in ComHelpers that handles falling back to ComWrappers when necessary. The simple implementation would be something like this:

internal static unsafe partial class ComHelpers
{
    internal static bool BuiltInComSupported { get; }
        = AppContext.TryGetSwitch("System.Runtime.InteropServices.BuiltInComInterop.IsSupported", out bool supported)
            ? supported
            : true;

    public static object GetObjectForIUnknown(IUnknown* unknown)
        => BuiltInComSupported
            ? Marshal.GetObjectForIUnknown((nint)unknown)
            : ComInterfaceMarshaller<object>.ConvertToManaged(unknown).OrThrowIfNull();
}

We should also consider a switch that allows forcing the ComInterfaceMarshaller and potentially picking it by default if the reflection based support for casting to [ComImport] interfaces is enabled. ("System.Runtime.InteropServices.Marshalling.EnableGeneratedComInterfaceComImportInterop" AppContext flag or "EnableGeneratedComInterfaceComImportInterop" in MSBuild)

The only issue with the ComInterfaceMarshaller is that presumes objects are free-threaded. We can deal with this by implementing our own IUnknownStrategy that wraps the GIT much as our AgileComPointer does. This would require deriving from StrategyBasedComWrappers and returning a new instance of the strategy by overriding GetOrCreateIUnknownStrategy().

[JeremyKuhne]

Fyi @jkoritzinsky

[weltkante]

not sure how much this is related, but there is desire of having AxHost work in trimming/NativeAOT scenarios. I'm currently doing handwritten bindings in #10583 to get the basics working for source generators - but while the ActiveX control itself currently can be (manually) written using COM source generators, WinForms itself (AxHost interaction) needs work done to drop classic COM interop

[JeremyKuhne]

ComHelpers.GetObjectForIUnknown now does this.