Suppose I have a native COM class A which implements an interface ITest,
and a native COM class B which features a member function taking a
parameter of type ITest. With normal COM Interop, I can call this
function in C# by explicitely casting to ITest as in

  someB.TestFunction((ITest) someA);

which also take care of the QueryInterface behind the scenes.

Suppose further that for some obscure reason a user wants to
use the library in a late-bound scenario using reflection.
I can load the assembly, locate types "typeA", "typeB",
"typeITest", and dynamically create instances of A and B
storing them in object variables "objA" and "objB".

But I don't know how to pass objA as an argument when
invoking the TestFunction on objB:

typeB.InvokeMember("TestFunction",
                   BindingFlags.InvokeMethod,
                   null,
                   objB,
                   new object[] {objA}); // ???

With the above code, the function complains that the argument
is not of type ITest. I tried something along the lines of

Guid guidITest  = typeITest.GUID;
IntPtr pUnk     = Marshal.GetIUnknownForObject(objA);
IntPtr pITest;
int HRESULT     = Marshal.QueryInterface(pUnk,ref guidITest,out pITest);
object objITest = Marshal.GetTypedObjectForIUnknown(pUnk, typeITest);

but I invariably end up with some object instance of type A (AClass),
although it looks like the QueryInterface returns meaningful pointers.

Any insights appreciated.