c++ MFC SDI DLL call method of OLE Server using IDispatch - c++

Question

I have written a c++ MFC DLL that brings up an SDI Application which is a very legacy OLE Server. (I have no choice about using this OLE Server so I have to make it work.)
I am accessing this c++ DLL from C#.
I have everything "working". I can call methods of the DLL, I have implemented C# delegates correctly, etc.
I can call methods of the OLE Server directly in C++ and export these so that my C# application call call them too. This is my "Hello World" for accessing the OLE Server functionality in its entirety from C#.
So far so good.
The next step is to make this C++ DLL as much of a "pass-through" as possible so that C# developers can write business logic around this OLE server. If changes or updates happen from the maker of the OLE Server, we do not want to have to update C++ code, we want to respond to the changes in C#.
So even though I can implement methods in C++ using the imported class from the OLEServer.tlb file and pass these through to C#, we do not want to ultimately do this. I am trying to call methods through the IDispatch interface instead and I am running into difficulties that I can't quite understand.
I am using Visual Studio 2010 for both unmanaged C++ and C#.
VARIANT LaserCat::LaserCatCommand(LPCTSTR* p_pMethodNameAndParamsInReverseOrder, UINT p_Count)
{
AFX_MANAGE_STATE(AfxGetStaticModuleState());
VARIANT result;
VARIANT* pResult = NULL;
VariantInit(&result);
HRESULT hr = NULL;
DISPID dispid;
const IID IID_ITriad = {0x60EE772D,0xE076,0x4F58,{0xA8,0xB4,0x2F,0x7A,0x29,0xBB,0x02,0x50}};
COleException* pError = NULL;
BOOL HasDispatch = theApp.pTriadView->pTriadItem->Catalog.CreateDispatch(IID_ITriad, pError);
LPDISPATCH iDisp = theApp.pTriadView->pTriadItem->Catalog.m_lpDispatch;
LPOLESTR Names[1] = {(LPOLESTR)L"GetInterfaceVersion"};
hr = iDisp->GetIDsOfNames(IID_NULL, Names, 1, LOCALE_SYSTEM_DEFAULT, &dispid);
if (hr != S_OK) return result;
DISPPARAMS* pParams = new DISPPARAMS();
short maj = 0;
short min = 0;
short* nMajor = &maj;
short* nMinor = &min;
VARIANTARG Args[2];
VariantInit(&Args[0]);
VariantInit(&Args[1]);
Args[0].piVal = nMinor;
Args[0].vt = VT_BYREF;
Args[1].piVal = nMajor;
Args[1].vt = VT_BYREF;
pParams->rgvarg = Args;
pParams->cNamedArgs = 0;
pParams->cArgs = 2;
pParams->rgdispidNamedArgs = NULL;
EXCEPINFO* pExcept = NULL;
UINT* pArgErrorIndex = NULL;
LPCTSTR Error = NULL;
hr = iDisp->Invoke(dispid, IID_NULL, LOCALE_SYSTEM_DEFAULT, DISPATCH_METHOD, pParams, pResult, pExcept, pArgErrorIndex);
if (pExcept != NULL || pArgErrorIndex != NULL || hr != S_OK)
{
Error = _T("Error");
return result;
}
result = *pResult;
return result;
}
The following line from above gives me a "Bad variable type" error:
hr = iDisp->Invoke(dispid, IID_NULL, LOCALE_SYSTEM_DEFAULT, DISPATCH_METHOD, pParams, pResult, pExcept, pArgErrorIndex);
BTW, I am sure that the code can be vastly improved, it has been years since I have written in C++ and this is just a "first kick" at getting this working so no real error handling etc.
I have toyed around with "Args[]" type and value with variations of "Type Mismatch" errors and "Null Reference Pointers"
The function that is imported by the .tlb file looks like this:
short GetInterfaceVersion(short * nMajor, short * nMinor)
{
short result;
static BYTE parms[] = VTS_PI2 VTS_PI2 ;
InvokeHelper(0x178, DISPATCH_METHOD, VT_I2, (void*)&result, parms, nMajor, nMinor);
return result;
}
Oh, although I am passing in the "pMethodNameAndParamsInReverseOrder" as a parameter, I am just hard-coding it to get this one simple method working. Once I have it working with this and a few other methods, I am planning on making this generic to handle any methods implemented by the COM interface via IDispatch.
Any help would be appreciated mostly in getting this working but I would also appreciate any pointers on improving the code, I have mouch to learn in C++
Thank-You!
BTW, If this helps clarify things, theApp.pTriadView->pTriadItem->Catalog is the COM class I am implementing
EDIT:
Thanks to #HansPassant (see first comment) I see what I was missing. Unfortunately I have hit a downstream result of fixing that. The VARIANT "pResult" is coming back empty. I will continue to hunt that down now but any thoughts would be welcome :)