PRB: Assertion or Problems Using DAO in a DLL

ID: Q149889

The information in this article applies to:

The Microsoft Foundation Classes (MFC), used with:
- Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1, 4.1b, 4.2, 4.2b, 5.0, 6.0

SYMPTOMS

This article provides some guidelines for using DAO in regular DLLs. In general, avoid performing any DAO construction, destruction, or operations inside the DLL's InitInstance or ExitInstance functions. Otherwise, you might see the following assertions:

Assertion failed in DAOCORE.CPP, line 36

(in Visual C++ 4.0)

Assertion failed in APPCORE.CPP, line 585 (in Visual C++ 4.1)

(in Visual C++ 4.1)

STATUS

This behavior is by design. MFC/DAO objects need to be created after CWinApp::InitInstance and destroyed before terminating DAO. By default, MFC attempts to terminate DAO within CWinApp::ExitInstance.

MORE INFORMATION

The AfxDaoTerm helper function terminates the DAO database engine. In applications, AfxDaoTerm is called automatically, but in DLLs, it must be explicitly invoked before the DLL's ExitInstance function.

Here are some general guidelines to follow:

Create any MFC/DAO objects after the DLL's InitInstance function. For additional information, please see the following article in the Microsoft Knowledge Base:
Q147629 PRB: Problems If ODBC or DAO in DLL's InitInstance or DLLMain

Destroy these objects before calling AfxDaoTerm.

Call AfxDaoTerm before calling the DLL's ExitInstance function.

Because the AfxDaoTerm helper function terminates the database engine, it must be called after all MFC/DAO objects have been destroyed.

AfxDaoInit

This function initializes the DAO database engine. In most cases, you don't need to call AfxDaoInit because the application calls it when it is needed. AfxDaoInit is called during the construction of the first MFC/DAO object.

AfxDaoTerm

This function terminates the DAO database engine. Typically, this function only needs to be called in a DLL; an application automatically calls AfxDaTerm when it is needed.

When DAO is initialized, MFC will set the m_lpfnDaoTerm pointer in the CWinApp class to point to AfxDaoTerm(). When CWinApp::ExitInstance is invoked, by default it invokes the value of m_lpfnDaoTerm if it is set.

In a Regular DLL, you need to call AfxDaoTerm() before the default ExitInstance is invoked because ExitInstance is invoked by DllMain. Visual C++ 4.0 had a documented bug where the .exe's ExitInstance would shut down DAO, causing an assertion when the Regular DLL's ExitInstance was called.

For additional information, please see the following article in the Microsoft Knowledge Base:

Q143084 FIX: Problems with Using the MFC DAO Classes in a .DLL or .OCX

All MFC/DAO objects in the DLL must be destroyed before the call to AfxDaoTerm. Be careful about the scope of local and global DAO objects. For example, the following code causes an assert:


   SomeExportedFunc( .. )
   {
      // Only call for regular MFC DLL's
      #ifndef _AFXDLL
      AFX_MANAGE_STATE(AfxGetStaticModuleState());
      #endif

      CDaoDatabase db;
      db.Open(..);
      db.Close();
      AfxDaoTerm();
   }

Because the DAO object db is a local variable, it remains in scope until SomeExportedFunc returns. The call to AfxDaoTerm causes an assertion because DAO terminates while db still has scope. Similarly, a global DAO object has scope throughout the life of the DLL, so a call to AfxDaoTerm also results in an assertion.

To ensure that your MFC/DAO objects are destroyed before calling AfxDaoTerm, avoid global objects and create local objects dynamically by using the new operator as in this example:


   SomeExportedFunc( .. )
   {
      // Only call for regular MFC DLL's
      #ifndef _AFXDLL
      AFX_MANAGE_STATE(AfxGetStaticModuleState());
      #endif

      CDaoDatabase *DB = new CDaoDatabase;
      pDB->Open(..);

      // do something

      pDB->Close();

      // Destroy the object with delete
      delete pDB;

      // can now safely terminate DAO
      AfxDaoTerm();
   }

A variation is to export special creation and termination functions as in the following example. The advantage of this method is that DAO is running and connections remain open during the life of the DLL. The disadvantage is that the user of the DLL is responsible for explicitly calling these special functions.


   SomeExportedInitializeDAO( .. )
   {
      // Only call for regular MFC DLL's
      #ifndef _AFXDLL
      AFX_MANAGE_STATE(AfxGetStaticModuleState());
      #endif

      m_pDB = new CDaoDatabase;
      m_pDB->Open(..);
   }

Then, to terminate MFC/DAO:


   SomeExportedDestroyObjectsAndTerminateDAO( .. )
   {
      // Only call for regular MFC DLL's
      #ifndef _AFXDLL
      AFX_MANAGE_STATE(AfxGetStaticModuleState());
      #endif

     m_pDB->Close();
     delete m_pDB;
     AfxDaoTerm();
   }

REFERENCES

For related information, see Technical Note 54. Technical Notes are available under MFC Technical Notes, under MFC Books Online.

For additional information on how to use AFX_MANAGE_STATE correctly, please see MFC Technical Note 58 (TN058) and the following article in the Microsoft Knowledge Base:

Q140850 HOWTO: Properly Export Functions Using the MFC Shared Library

AFX_MANAGE_STATE should be used in Regular DLLs only.

Additional query words:

Keywords : kbcode kbDAO kbDatabase kbMFC kbVC kbVC400 kbVC410 kbVC420 kbVC500 kbVC600
Version : winnt:
Platform : winnt
Issue type : kbprb