FIX: MFC DLLs Don't Unregister Window Classes (108113)

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);
}