Microsoft KB Archive/143084

= FIX: Problems with Using the MFC DAO Classes in a .DLL or .OCX =

Article ID: 143084

Article Last Modified on 10/24/2003

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APPLIES TO


 * Microsoft Foundation Class Library 4.2, when used with:
 * Microsoft Visual C++ 4.0 Standard Edition

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This article was previously published under Q143084



SYMPTOMS
Two errors can occur when using the MFC DAO database classes in a DLL. Depending upon the context of how you use the MFC DAO classes in the DLL, you can receive an assertion on line 36 of the Daocore.cpp or you may receive an access violation.



CAUSE
The root of the problem exists in how MFC is terminating DAO. MFC calls AfxDaoTerm from the exit instance of CWinApp. This is fine for applications but isn't a good thing to do for DLLs because ExitInstance is called from the DLL_PROCCESS_DETACH case. Use of OLE and specifically DAO should be avoided in the ExitInstance of a DLL.

Another problem with the MFC DAO code is that it stores the pointer to the database engine on a global scope so that if one regular DLL that links to MFC dynamically closes and calls AfxDaoTerm, it closes the database engine for any other DLLs which use DAO and link dynamically to MFC.

The line 36 assertion is caused by the fact that an exception gets thrown in AfxDaoTerm when trying to call Release on the database engine. The exception prohibits the setting of m_pDAOEngine pointer back to NULL. Again, this problem occurs because MFC incorrectly calls DAO in ExitInstance of a DLL.

Here is the code in AfxDaoTerm: // Clean up engine object if necessary if (pDaoState->m_pDAODBEngine != NULL) {           pDaoState->m_pDAODBEngine->Release; pDaoState->m_pDAODBEngine = NULL; } You can see that if an exception occurs in Release, the m_pDAODBEngine pointer is not set to NULL. Thus, the assert on line 36 of Daocore.cpp will occur: ASSERT(m_pDAODBEngine == NULL); An access violation can occur if you perform the following sequence:


 * 1) Dynamically load DLL1 and use MFC DAO classes.
 * 2) Dynamically load DLL2 and use MFC DAO classes.
 * 3) Close DAO objects and unload DLL1.
 * 4) Do something with DAO in DLL2.

This occurs because the DLL1 has already shut down the DAO components.

There may be other scenarios that can cause other access violations to occur. For example, do not create global MFC DAO objects within a DLL. The resolution section below doesn't address this problem because it is something that should not be done.



RESOLUTION
To work around the problems with DAO in a DLL, do one of the following:


 * Link all DLLs using the static version of MFC, except in the case of an .ocx that requires the shared MFC DLL.

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 * Construct and Open a CDaoWorkspace object in the calling application so that the DAO engine is initialized and terminated in the application rather than in the any of your DLLs.

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 * Use the dbDAO C++ classes provided with the DAO SDK, which is installed from the Visual C++ CD-ROM compact disc.



STATUS
Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug was corrected in Visual C++ 4.1.

