Q121218: FIX: pvarResult Should Be NULL When vtRet==VT_EMPTY

Article: Q121218
Product(s): Microsoft C Compiler
Version(s): 1.5,1.51,2.0
Operating System(s): 
Keyword(s): kbole kbMFC kbVCkbbuglist kbfixlist
Last Modified: 25-JUL-2001

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The information in this article applies to:

- Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, versions 1.5, 1.51, 2.0 
   - *EDITOR Please do not choose this product*Microsoft Visual C++ 32-bit Edition* use 241, 265, 225, version 2.0 
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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 kbbuglist kbfixlist
Technology        : kbAudDeveloper kbMFC
Version           : :1.5,1.51,2.0
Issue type        : kbbug
Solution Type     : kbfix

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