Adding a COM DLL from a tlb/dll via Add New Class wizard (VC 6.0) and accessing methods

Posted on 2004-10-17
Last Modified: 2013-11-25
I have created a ATL/Com DLL (rfcomdll.dll) and registered it. It has one method (at this time) called DispatchReq (code excerpt below).

STDMETHODIMP CProcessREQ::DispatchReq(BSTR data, int terminal, BSTR *Message)
         CComBSTR str;
         str = "Hello";
         str += data;
        *Message = str.Detach();
        return S_OK;

The COM object has the following registry....

RFComDll.ProcessREQ.1 = s 'ProcessREQ Class'
 CLSID = s '{50FA39B5-290B-46CC-B950-AD65BEFC661D}'
RFComDll.ProcessREQ = s 'ProcessREQ Class'
CLSID = s '{50FA39B5-290B-46CC-B950-AD65BEFC661D}'
CurVer = s 'RFComDll.ProcessREQ.1'
NoRemove CLSID
ForceRemove {50FA39B5-290B-46CC-B950-AD65BEFC661D} = s 'ProcessREQ Class'
ProgID = s 'RFComDll.ProcessREQ.1'
VersionIndependentProgID = s 'RFComDll.ProcessREQ'
ForceRemove 'Programmable'
InprocServer32 = s '%MODULE%'
val ThreadingModel = s 'Both'
'TypeLib' = s '{C1B9CF16-CA21-4677-A492-FA2616426DA4}'

I created a SDI VC++ exe project that I want to access the COM object from.

I added the COM class via the Add New Class wizard (using the tlb - type library) which has created the class (cpp & h files) in my project. Below is an excerpt of the cpp file with the helper code that class wizard created.

CString IProcessREQ::DispatchReq(LPCTSTR data, long terminal)
      CString result;
      static BYTE parms[] =
            VTS_BSTR VTS_I4;
      InvokeHelper(0x1, DISPATCH_METHOD, VT_BSTR, (void*)&result, parms,
            data, terminal);
      return result;

I have the following code in a the main view of my project called MyClient...

CString s;
IProcessREQ r;
COleException *e = new COleException;

s = r.DispatchReq("MyData",1);

This code is failing partially because the m_lpDispatch class member is NULL among other problems. I badly assumed that by using class wizard I could access the method without issue. What is the best way to do this?

Question by:carlmahon
  • 2
LVL 12

Assisted Solution

OnegaZhang earned 250 total points
ID: 12336130
call AfxOleInit() before CreateDispatch

welcome to

Author Comment

ID: 12336356
The MFC framework must be calling AfxOleInit() already.  If I call AfxOleInit() before CreateDispatch() it causes an assertion error and I can see in the debugger that it has already been called at some point during program initialization. So calling that function a second time is not helping.

LVL 16

Accepted Solution

nonubik earned 250 total points
ID: 12336458
Note   If AfxOleInit is called from an MFC DLL, the call will fail. The failure occurs because the function assumes that, if it is called from a DLL, the OLE system was previously initialized by the calling application.

You should try calling CoInitialize(0) or CoInitializeEx(0, COINIT_APARTMENTTHREADED) before using COM. And for each successfull CoInitialize, call CoUninitalize when done with COM. Be carefull not to call them from DllMain (or CWinApp derived class InitInstance/Exitinstance methods).

Author Comment

ID: 12341158
After some testing here is what I found after I watched the code in the debugger...

When compiling the COM dll, VCC was performing the registration after the DLL was compiled. I was not getting any errors in the compile output so I assumed it registered. I wanted to check that the registration was really happening so I used regsvr32 and component services to manually register the component.  Both of those failed (even if the DLL was previously unregistered using regsvr32 /u). After some research on the Microsoft KB I discovered that the atl.dll had not been registered on my system which is required for VC++6.0. (Not sure whay it was ever unregistered). After I registered the atl.dll I could then register and access the component.

My original coding worked without using the CoInitialize(0) answer provided by nonubik, but it may also have been automatically called within MFC framework, I do not know. Since I did not indicate that there could have been COM registration issue I will split the points as an assist from both. At least once I started looking at those functions in the debugger it led me to realize that the DLL just wasn't there.


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