How set session variable using IDISPATCH

Since MS cannot make QSRules binary compatible between service packs I am trying to late bind my C++ code through Idispatch. I can make everything work except setting the session variable. When I make the call using my code I always get "parameter not found"

The idl is: [id(0x68030020), propput] HRESULT Variable([in, out] long* I, [in, out] VARIANT* );

And my attempt at the call is:

long lngSessionVarNum = 1;

VARIANT msg; VariantInit(&msg); msg.vt = VT_BSTR;; msg.bstrVal = L"Hello World";

CComVariant varVal1,varVal2;

varVal2.SetByRef(&msg); varVal1.SetByRef(&lngSessionVarNum);

CComVariant varArgs[2] = { varVal2, varVal1 };

DISPPARAMS dispparams = { &varArgs[0], NULL, 2, 0};

hr = m_oServer->Invoke(dispid, IID_NULL, LOCALE_USER_DEFAULT, DISPATCH_PROPERTYPUTREF, &dispparams, NULL, NULL, NULL);

This always returns parameter not found. I have also tried DISPATCH_PROPERTYPUT and that does not work either.

Thanks

KB