Microsoft KB Archive/108113

= FIX: MFC DLLs Don't Unregister Window Classes =

Article ID: 108113

Article Last Modified on 11/21/2006

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APPLIES TO


 * Microsoft Foundation Class Library 4.2, when used with:
 * Microsoft Visual C++ 1.0 Professional Edition

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This article was previously published under Q108113





SYMPTOMS
If an MFC dynamic-link library (DLL) is loaded with LoadLibrary and later released with FreeLibrary, it will not be possible to load the library a second time during the same invocation of the main program. The library fails during initialization.



CAUSE
Four window classes are registered by the MFC initialization code. Windows NT 3.1 does not automatically unregister window classes that were registered by a dynamically loaded DLL when the DLL's reference count reaches zero and the DLL is removed from memory. If the DLL is loaded a second time, the RegisterClass call in the initialization code fails because the classes are still registered.



RESOLUTION
The workaround is to unregister the window classes in the destructor for the DLL CWinApp object. The sample code below is an example of the code that is needed.

If you register your own window class, you will also need to unregister that window class. The class name is returned as a char * by the AfxRegisterWndClass function.



STATUS
Microsoft has confirmed this to be a bug in the products listed at the beginning of this article. This problem was corrected in MFC 3.0, included with Visual C++ 32-bit Edition, version 2.0.

Sample Code
/* This is a code fragment and is not meant to be used alone. It will * not compile. * * CMyDll is derived from CWinApp and is the application object for the * DLL. */ CMyDll::~CMyDll {  UnregisterClass("AfxWnd", m_hInstance); UnregisterClass("AfxControlBar", m_hInstance); UnregisterClass("AfxMDIFrame", m_hInstance); UnregisterClass("AfxFrameOrView", m_hInstance); }

Additional query words: 1.00 2.00 2.10 USRDLL

Keywords: kbbug kbdll kbfix kbvc200fix KB108113

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