C# Reflection inconsistency with COM objects

Having spent the last few days reading everything I can find about C# reflection on COM objects, trying many experiments in code, and analyzing sample code to try and improve my understanding, I am now forced to admit that I just don't know enough, so I am requesting help from the Community.

I need to be able to access and update the properties of a late bound COM object that is wrapped as System._COMObject. I tried all the standard refection stuff without success and I looked through using IDispatch, but I'm not comfortable with using the pointers involved, so I'm hoping I have missed something pretty simple in the normal interface. I found papers on MSDN that DO show how to do what I need, but all the examples are in C++ and it is over my head.

It would be really helpful if someone could explain why the following simple C# code just doesn't work as expected:

            try
        {
            // late binding:
            // localCB is a COM object (System._COMObject) created by Activator.CreateInstance() from 
            // the ProgID of a registered COM .DLL. 
            // 
            // The original .DLL has a string PROPERTY called 
            // "TESTEXTERNAL1". localCB has an IDispatch Interface.  The original COM .DLL has a separate Typelib,
            // and, although I did not register the typelib separately, I can see from OLEView on the COM object
            // that  the GUID for the typelib is included in it.

            // Here's the code that is puzzling me...
             var vv = localCB.GetType().InvokeMember("TESTEXTERNAL1", BindingFlags.GetProperty, 
                  null, localCB, null);
            string rt = vv.ToString();
            // works exactly as expected and returns the value of TESTEXTERNAL1 -  OK.

            // now try and update the SAME PROPERTY, on the SAME COM object...
            Parameters = new Object[1];
            Parameters[0] = "Hello, World!";
            localCB.GetType().InvokeMember("TESTEXTERNAL1", BindingFlags.SetProperty, 
                  null, localCB, Parameters);
            // throws an (inner) exception: HRESULT 0x8002003 DISP_E_MEMBERNOTFOUND !!!

        }
        catch (Exception xa)
        {
            string xam = xa.Message;
        }

Is it unreasonable to expect an object that has already found and provided a property, to be able to update the same property? Is there some "alternative update" strategy that I am not aware of?

Many thanks for any help,

Pete.