PRB: FromIDispatch Returns NULL for OLE Control

Last reviewed: July 31, 1997
Article ID: Q138414

2.00 2.10 | 1.50 1.51 1.52 1.52a WINDOWS NT | WINDOWS kbprg kbtshoot kbprb

The information in this article applies to:

The Microsoft Foundation Classes (MFC) included with:

    - Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1
    - Microsoft Visual C++ for Windows, versions 1.5, 1.51, 1.52, 1.52a

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.

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.

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).

STATUS

This behavior is by design.

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.

Additional reference words: 2.00 2.10 1.50 1.51 1.52 1.52a
KBCategory: kbprg kbtshoot kbprb
KBSubcategory: CDKIss
Technology : kbMfc
Version : 2.00 2.10 | 1.50 1.51 1.52 1.52

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Last reviewed: July 31, 1997
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