Pass a COM-HANDLE to a C# method

I need to pass a COM-HANDLE (specifically to an Outlook UserProperty object), as a parameter to an External C# dll.

Within C# this is easy, simply pass it as "Object", but from OE the COM-HANDLE doesn't map to object and I can't cast it to "System.Object" pass.

I have also tried changing the signature of the C# method to accept the UserProperty as the Interop type, but once again that fails.

Any suggestions as to how to do this ?