HOWTO: Using the MFC Database Classes in Console Applications

• The Microsoft Foundation Classes (MFC), included with:
  • Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 2.2, 4.0, 4.1, 4.2, 5.0, 6.0

SUMMARY

The MFC database classes found in Visual C++ versions 1.x, 2.x, 4.0, and 4.1 were not written to be used in a console application. However, with some modification, you can use the 32-bit database classes in a console application. The techniques presented here apply to both the MFC ODBC and MFC DAO classes.

Starting with Visual C++ 4.2, the MFC ODBC classes were rendered safe for use within console applications. Many of the internal dependencies listed below are no longer present in the 4.2 and later MFC ODBC class libraries.

MORE INFORMATION

Both the MFC ODBC and MFC DAO classes have dependencies upon non-Database MFC classes, particularly with the existence of a global CWinApp-derived object. The following list presents these dependencies along with suggestions for working around them.

For either MFC ODBC or MFC DAO, it is very important to test your code not only for successful execution, but for graceful recovery on unexpected conditions (such as the database not being in an expected path). The majority of the "dependencies" these classes have on MFC come from error recovery and reporting code within the database classes.

It is also important to remember that the Visual C++ MFC database classes released through version 4.1 were not intended or tested for use in a console application.

Both MFC ODBC and MFC DAO Classes

• Exception Handling
  
  Microsoft recommends that you wrap exception handling code around each of your calls to an MFC Database class method.



• Exception Handling and Resource Handles
  
  Internally, MFC ODBC uses AfxThrowDBException to throw exceptions, and MFC DAO uses AfxThrowDaoException() to throw exceptions. Both functions can result in a call to CString::LoadString(), which requires a resource handle for the application. In addition, the MFC DAO method CDaoException::GetErrorMessage(), used to get more information about a DAO error, also calls CString::LoadString().
  
  Within CString::LoadString() is a call to AfxLoadString(), which calls AfxGetResourceHandle(), which expects a valid resource handle to be defined for the EXE.
  
  To work around this, define a valid resource handle for your application as shown in this code:

  
        hMod = ::GetModuleHandle( "MyApp.Exe" );
        ASSERT( hMod );
        AfxSetResourceHandle( hMod ); 

MFC ODBC Classes

• Use of AfxGetApp() in the MFC ODBC Classes
  
  In synchronous mode when an error is encountered, CDatabase::Open() attempts to get a pointer to a global CWinApp-derived object and assert if it is not found. To avoid this, you can:
  
  
  • Invoke CDatabase::SetSynchronousMode( TRUE ) to disable asynchronous processing.
    
    -or-
  
  
  
  • Create a global instance of a CWinApp-derived class in your application similar to the one shown below. This will give you a global CWinApp object in your console application without actually invoking MFC in any way.

    
             class CMyApp : public CWinApp
             {
             public:
                 virtual BOOL InitInstance( ) { return FALSE };
             };
    
             CMyApp theApp; 

  
  
  
  This dependency was removed in Visual C++ 4.2. Use of Synchronous mode was also removed, as was the OnWaitForDataSource and InWaitForDataSource methods for both CDatabase and CRecordset.



• Avoiding the Need for a Valid Windows Handle
  
  The CDatabase::Open() method also attempts to acquire a valid Windows handle to pass to SQLDriverConnect. If the application using the classes can't provide one, the GetDesktopWindow method is called. This handle is used by the ODBC API call SQLDriverConnect as a parent handle to create a dialog if the database connection information provided is incomplete or inaccurate.
  
  It is possible to prevent this dialog from being displayed. For information on how to do it, please see the following article in the Microsoft Knowledge Base:

  Q150552 How to Avoid ODBC Login Dialog When Using MFC

MFC DAO Classes

• Use of AfxGetApp() in the MFC DAO Classes
  
  The initialization of a CDaoWorkspace object -- whether explicitly or implicitly by way of creation of a CDaoDatabase or CDaoRecordset object -- forces a call to AfxDaoInit(). AfxDaoInit() attempts to ensure that DAO is properly terminated by setting the m_lpfnDaoTerm member of CWinApp to point to the AfxDaoTerm() function.
  
  If you do not have a global instance of a CWinApp-derived class, you need to call AfxDaoTerm() yourself to terminate DAO properly. For more information on the use of AfxDaoTerm(), please see the following article in the Microsoft Knowledge Base:

  Q145992 PRB: Global MFC DAO Objects Cause Assertions

Additional query words: kbvc200 kbvc210 kbvc220 kbvc400 kbvc410 kbvc420 kbvc500 kbvc600

Keywords : kbprg kbusage kbDAO kbDatabase kbMFC kbODBC kbVC
Version : 2.0 2.1 2.2 4.0 4.1 4.2 5.0 6.0
Platform : NT WINDOWS
Issue type : kbhowto