Knowledge Base

A failure may occur when you attempt to use object linking and embedding (OLE) within a Microsoft Foundation Classes (MFC) regular dynamic link library (DLL)

Article ID: 154320

Article Last Modified on 11/21/2006

Microsoft Foundation Class Library 4.2, when used with:
- Microsoft Visual C++ 4.2 Enterprise Edition
- Microsoft Visual C++ 4.2 Professional Edition
- Microsoft Visual C++ 5.0 Enterprise Edition
- Microsoft Visual C++ 5.0 Professional Edition
- Microsoft Visual C++ 6.0 Enterprise Edition
- Microsoft Visual C++ 6.0 Professional Edition
- Microsoft Visual C++ 6.0 Standard Edition

This article was previously published under Q154320

SYMPTOMS

When you develop a Microsoft Foundation Classes (MFC) regular dynamic link library (DLL), you may want to use object linking and embedding (OLE) without exposing an OLE object, such as displaying a dialog box from within the DLL that contains an OLE control. A failure may occur when you call the DoModal function in the dialog box creation if you used the AfxOleInit or AfxEnableControlContainer function to initialize OLE within the DLL. Typically, this is done by calling the functions in the InitInstance method of the CWinApp derived class.

CAUSE

In a MFC regular DLL, AfxOleInit does not initialize OLE because MFC cannot uninitialize OLE in the DLL_PROCESS_DETACH method of the ExitInstance function. In this case, OLE may already have been unloaded. The decision to not initialize OLE is by design. The bug is that AfxOleInit returns TRUE after setting m_bNeedTerm to -1. According to the Visual C++ documentation, when AfxOleInit returns TRUE this means that OLE was initialized, and that is not the case in a DLL.

RESOLUTION

In order to uninitialize OLE when it is initialized in the DLL, this requires exported methods for initialization and uninitialization that the client has to call because the uninitialization has to occur before the CWinApp::ExitInstance function, which is a part of DLL_PROCESS_DETACH.

A better solution is to initialize and uninitialize OLE in the client application. This can be done in MFC by calling AfxOleInit in the application's InitInstance method of the CWinApp derived class. MFC then uninitializes OLE by exiting the application.

STATUS

Microsoft has confirmed that this is a bug in the Microsoft products that are listed in the "Applies to" section.

Additional query words: 0x800401f0 CreateDlgControls Dialog creation

Keywords: kboleapp kbtshoot kbbug kbdll kbdocfix kbnofix KB154320