[API Proposal]: Support getting IUnknown for ComWrappers RCWs #79679
Description
Background and motivation
For COM interop we expect to be able to cast RCWs to arbitrary [ComImport] interfaces. We use this and Type.IsComObject to implement TypeDescriptor functionality.
We can reimplement to support both legacy COM Interop and ComWrapppers if we have the ability to get IUnknown* for any ComWrappers generated RCWs.
Without this support we cannot enable trimming for WinForms without breaking critical features such as drag/drop/cut/paste and accessibility. This also prevents providing COM ComWrappers support with TypeDescriptor (which WinForms owns).
API Proposal
namespace System.Runtime.InteropServices;
public abstract class ComWrappers
{
+ public static bool TryGetComInstance(object obj, out void* unknown);
}API Usage
public static bool TryCreateComTypeDescriptor(object obj, out ComTypeDescriptor? descriptor)
{
void* unknown;
if (obj.GetType().IsCOMObject)
{
unknown = (void*)Marshal.GetIUnknownForObject(obj);
}
else
{
ComWrappers.TryGetComInstance(obj, out unknown);
}
return unknown is not null ? new ComTypeDescriptor(unknown) : null;
}Alternative Designs
One alternative (or addition) is to change all possible Marshal APIs that provide interface pointers to support ComWrappers RCWs:
namespace System.Runtime.InteropServices;
// Existing interface pointer methods
public static class Marshal
{
public static IntPtr GetComInterfaceForObject(object! o, Type! T);
public static IntPtr GetComInterfaceForObject(object! o, Type! T, CustomQueryInterfaceMode mode);
public static IntPtr GetComInterfaceForObject<T, TInterface>([DisallowNull] T o);
public static IntPtr GetComInterfaceForObjectInContext(object o, Type t);
public static IntPtr GetIDispatchForObject(object! o);
public static IntPtr GetIDispatchForObjectInContext(object o);
public static IntPtr GetIUnknownForObject(object! o);
public static IntPtr GetIUnknownForObjectInContext(object o);
}I don't know how plausible this is, but it would make writing consuming code much easier.
Risks
No response