How to get pointer to IUnknown in C#

Question

.NET interop wraps COM objects into .NET objects (runtime-callable wrappers, RCWs), which hide the usual interface querying. In order to register a COM object representing a filter graph with the Running Objects Table, I need the (native) address of its IUnknown interface (see How can I reverse engineer a DirectShow graph?).

So the question is: How can I get a COM object's IUnknown pointer in C#?

Answer 1

Use Marshal.GetIUnknownForObject().