PRB: FromIDispatch Returns NULL for OLE Control

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This article was previously published under Q138414

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SYMPTOMS

Calling CCmdTarget::FromIDispatch() inside a member function of a class derived from COleControl or attempting to call FromIDispatch() passing an IDispatch pointer belonging to an OLE Control, will always return NULL.

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CAUSE

CCmdTarget::FromIDispatch() does the following test to determine if the passed IDispatch pointer belongs to a CCmdTarget-derived class or not:

    COleDispatchImpl dispatch;

    ASSERT(*(DWORD*)&dispatch != 0);    // null vtable ptr?
    if (*(DWORD*)lpDispatch != *(DWORD*)&dispatch)
        return NULL;    // not our Idispatch*

The problem here is that classes derived from COleControl don't use the COleDispatchImpl implementation of IDispatch directly. They have their own implementation using BEGIN_INTERFACE_PART and END_INTERFACE_PART that delegates to the COleDispatchImpl object in the CCmdTarget base class. Because of this, the v-tables in the previous test won't match and the function will return NULL.

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RESOLUTION

It is possible to access the object of the class derived from COleControl given a valid IDispatch pointer. The object can be accessed from another control or outside of a control with the aid of a helper control. A helper control would be necessary because the following formula relies on details that are internal to a COleControl class.

To access the object of the class derived from COleControl successfully, you need to use a formula similar to this one:

   COleControl * pCtrl = (COleControl*)
       ((BYTE*)lpDisp - m_xDispatch.m_nOffset);

In this formula, lpDisp is a valid LPDISPATCH, m_xDispatch is the XDispatch object member of COleControl that contains the control's implementation of IDispatch, and m_nOffset is the offset generated by the INIT_INTERFACE_PART macro for IDispatch.

m_xDispatch is generated by the END_INTERFACE_PART macro as follows:

   #define END_INTERFACE_PART(localClass) \ 
      } m_x##localClass; \ 
      friend class X##localClass; \

In this case, localClass is "Dispatch" so m_x##localClass becomes m_xDispatch.

m_nOffset is generated by the INIT_INTERFACE_PART macro as follows:

    #ifndef _AFX_NO_NESTED_DERIVATION
    #define INIT_INTERFACE_PART(theClass, localClass) \ 
          size_t m_nOffset; \ 
             INIT_INTERFACE_PART_DERIVE(theClass,localClass) \ 

    #define INIT_INTERFACE_PART_DERIVE(theClass, localClass) \ 
          X##localClass() \ 
        { m_nOffset = offsetof(theClass, m_x##localClass); } \ 
m_nOffset becomes offsetof(COleControl, m_xDispatch).

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STATUS

This behavior is by design.

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MORE INFORMATION

This difference between CCmdTarget and COleControl's implementation of IDispatch disappears in Visual C++ version 4.0. COleControl uses COleDispatchImpl in version 4.0 and therefore, FromIDispatch() will work.

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APPLIES TO

Microsoft Foundation Class Library 4.2, when used with:

Microsoft Visual C++ 2.0 Professional Edition
Microsoft Visual C++ 4.1 Subscription
Microsoft Visual C++ 1.5 Professional Edition
Microsoft Visual C++ 1.51
Microsoft Visual C++ 1.52 Professional Edition

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kbprb KB138414

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Retired KB Content Disclaimer

This article was written about products for which Microsoft no longer offers support. Therefore, this article is offered "as is" and will no longer be updated.

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