FIX: pvarResult Should Be NULL When vtRet==VT_EMPTY

ID: Q121218

The information in this article applies to:

Microsoft Foundation Classes (MFC), used with:
- Microsoft Visual C++ for Windows, versions 1.5, 1.51, 2.0
- Microsoft Visual C++ 32-bit Edition, version 2.0

SYMPTOMS

A call to COleDispatchDriver::InvokeHelper causes the server to generate an error when the return type is VT_EMPTY. The nature of the error depends on the automation server. For example, the Word.Basic automation server causes the following error on some methods that have a return type of void (_DEBUG build):

Assertion Failed Line 315, OLEDISP2.CPP

By using the debugger to examine the excepInfo structure, you'll see that the btrDescription member contains this message:

Non Function called as function.

CAUSE

The OLE version 2.0 documentation indicates that in a call to IDispatch- >Invoke the pvarResult parameter should be NULL if the caller does not expect a return value. However the MFC implementation always passes a non- NULL pointer to an empty variant structure. In most cases an automation server will simply ignore pvarResult when it doesn't intend to return a value. But some servers will actually perform error checking on this parameter and will generate an error if it is non-NULL.

RESOLUTION

To work around this problem, derive your own class from COleDispatchDriver, such as CMyOleDispatchDriver, and create your own InvokeHelperVoid function. Then call your own InvokeHelperVoid function instead of InvokeHelper when you need to call a method that has no return value.

To implement this work around, create two new functions in your COleDispatchDriver derived class:


   CMyOleDispatchDriver::InvokeHelperVVoid
   CMyOleDispatchDriver::InvokeHelperVoid

Implement these functions by copying the implementations of InvokeHelperV and InvokeHelper from OLEDISP2.CPP.

Then modify the following line in InvokeHelperVoid from:


   InvokeHelperV(dwDispID, wFlags, vtRet, pvRet, pbParamInfo,
     argList);

to:


   InvokeHelperVVoid(dwDispID, wFlags, vtRet, pvRet, pbParamInfo,
     argList);

Then modify the following lines in InvokeHelperVVoid from:


   HRESULT hr = m_lpDispatch->Invoke(dwDispID, IID_NULL, 0, wFlags,
      &dispparams, &vaResult, &excepInfo, &nArgErr);

to:


   HRESULT hr = m_lpDispatch->Invoke(dwDispID, IID_NULL, 0, wFlags,
            &dispparams, NULL, &excepInfo, &nArgErr);

On some servers, you may also need to re-implement the COleDispatchDriver::SetProperty function and replace it with your own SetPropertyVoid function by copying the implementation from OLEDISP2.CPP. Just as for the InvokeHelper function, you will need to change it to call InvokeHelperVVoid instead of InvokeHelperV.

STATUS

Microsoft has confirmed this to be a bug in the products listed at the beginning of this article. This bug was corrected in Visual C++ 1.52 and Visual C++ 2.1.

Additional query words: 1.50 2.00 2.50 2.51 3.00

Keywords : kbole kbMFC kbVC
Version : :1.5,1.51,2.0
Platform : NT WINDOWS
Issue type :