HOWTO: Calling DAO Directly from MFC to Change a User's Password

• The Microsoft Foundation Classes (MFC) included with: - Microsoft Visual C++, 32-bit Edition, version 4.0, 4.1

SUMMARY

The MFC DAO database classes offer a subset of the Data Access Object Hierarchy. It is possible to manipulate the entire hierarchy with the help of OLE DAO interfaces available within the MFC Library. This article offers a demonstration of how to manipulate these interfaces in order to change the password of a user object found in the Users collection.

NOTE: Beginning with Visual C++ version 4.2, you will find this information and more in MFC Technical Note #54 in the Visual C++ online documentation.

MORE INFORMATION

The OLE interfaces for each object in the DAO Access Object Hierarchy are defined in the file found in the \Msdev\Include directory. These interfaces provide methods that allow you to manipulate the entire DAO Hierarchy. For many of the methods in the DAO Interfaces, you will need to manipulate a BSTR object (a length-prefixed string used in OLE Automation). The BSTR object typically is encapsulated within the VARIANT data type. The MFC class COleVariant itself inherits from the VARIANT data type.

Depending on whether you build your project for ANSI or UNICODE, the DAO interfaces will return ANSI or UNICODE BSTRs. There are two macros (V_BSTR and V_BSTRT) that are useful for assuring that the DAO interface gets the BSTR of the expected type.

V_BSTR will extract the bstrVal member of a COleVariant. This macro is typically used when you need to pass the contents of a COleVariant to a method of a DAO interface. The following code fragment shows both the declarations and actual use for two methods of DAOUser that take advantage of the V_BSTR macro:

  COleVariant varOldName;
  COleVariant varNewName( _T("NewUser"), VT_BSTRT );

  // Code to assign pUser to a valid value omitted
  DAOUser *pUser = NULL;

  // These method declarations were taken from Dbdaoint.h
  // STDMETHOD(get_Name) (THIS_ BSTR FAR* pbstr) PURE;
  // STDMETHOD(put_Name) (THIS_ BSTR bstr) PURE;

  DAO_CHECK( pUser->get_Name ( &V_BSTR(&varOldName) ) );
  DAO_CHECK( pUser->put_Name ( V_BSTR (&varNewName) ) );

   ARTICLE-ID: Q140599
   TITLE     : PRB: MFC DAO Functions Accepting COleVariant Strings
               May Fail

  COleVariant varName( _T( "MyName" ), VT_BSTRT );

  CString str = V_BSTRT( &varName );

The "Sample Code" section of this article uses these techniques to demonstrate calling the DAO OLE interfaces to change a user's password. This is done by manipulating two interfaces in particular: DAOUser and DAOUsers. DAOUsers is a collection of user objects, and like many of the collections inherits from the _DAODynaCollection interface. This interface offers two methods:

• Append
• Delete

• get_Count
• Refresh
• _newEnum

Finally, in order to change the user's password in this example, you will need to create a user named NewUser with a password of PassWord. The easiest way to do this is to use Microsoft Access. Note that the default password for a newly created user in Microsoft Access is blank. You will need to log in as that user in order to set the password. For more information, please see the Microsoft Jet Database Engine Programmer's Guide, Chapter 10, "Managing Security."

Sample Code

   // The following code presents several functions you can use to manage
   // MFC DAO classes in a way that allows you to call the DAO interfaces.
   // It then shows how to use those functions within InitInstance().

   // Set the system database that the DAO database engine will use:
   void AfxDaoSetSystemDB(CString & strSystemMDB)
   {
     COleVariant varSystemDB(strSystemMDB,VT_BSTRT );

     // Initialize DAO for MFC:
     AfxDaoInit();
     DAODBEngine* pDBEngine = AfxDaoGetEngine();

     ASSERT(pDBEngine != NULL);

     // Call put_SystemDB method to set the system database for DAO engine:
     DAO_CHECK( pDBEngine->put_SystemDB( varSystemDB.bstrVal ) );
   }

   void AfxDaoSetDefaultUser(CString & strUserName, CString & strPassword)
   {
     COleVariant varUserName( strUserName, VT_BSTRT );
     COleVariant varPassword( strPassword, VT_BSTRT );

     DAODBEngine* pDBEngine = AfxDaoGetEngine();
     ASSERT(pDBEngine != NULL);

     // Set default user:
     DAO_CHECK( pDBEngine->put_DefaultUser(varUserName.bstrVal));

     // Set default password:
     DAO_CHECK( pDBEngine->put_DefaultPassword(varPassword.bstrVal));
   }

   void AfxDaoChangePassword(CString & strUserName, CString &
   strOldPassword, CString & strNewPassword)
   {

       // Create(Open) Workspace:
       CDaoWorkspace   wsp;
       CString         strWspName     = _T("Temp Workspace");

       wsp.Create( strWspName, strUserName, strOldPassword );
       wsp.Append();

       // Determine how many objects there are in the Users collection:
       short       nUserCount;
       short       i;
       DAOUser     *pUser  = NULL;
       DAOUsers    *pUsers = NULL;

       // Side-effect is implicit OLE AddRef() on DAOUser object:
       DAO_CHECK( wsp.m_pDAOWorkspace->get_Users( &pUsers ) );

       // Side-effect is implicit OLE AddRef() on DAOUsers object:
       DAO_CHECK( pUsers->get_Count( &nUserCount ) );

   TRACE( "# of users in collection = %d\n", nUserCount );

       // Traverse through the list of users and change password for userid
       // used to create/open the workspace:
       for( i = 0; i < nUserCount; i++ )
       {
           COleVariant varIndex( i, VT_I2 );
           COleVariant varName;

           // Retrieve information for user i
           DAO_CHECK( pUsers->get_Item( varIndex, &pUser ) );

           // Retrieve name for user i
           DAO_CHECK( pUser->get_Name( &V_BSTR(&varName)) );

           TRACE( "User # %d Name = %s\n", i, V_BSTRT(&varName) );

           CString strTemp = V_BSTRT(&varName);

           // If there is a match, change the password:
           if( strTemp == strUserName )
           {
               COleVariant varOldPwd( strOldPassword, VT_BSTRT );
               COleVariant varNewPwd( strNewPassword, VT_BSTRT );

               DAO_CHECK( pUser->NewPassword( V_BSTR( &varOldPwd ),
                                              V_BSTR( &varNewPwd ) ) );

               TRACE( "\t Password is changed\n" );
           }
       }

       // Clean up: decrement the usage count on the OLE objects:
       pUser->Release();
       pUsers->Release();

       wsp.Close();
   }

   BOOL CMyApp::InitInstance()
   {
     // Specify path to the Microsoft Access system database:
     CString strSystemDB =
        _T("c:\\Program Files\\MSOffice\\access\\System.mdw");

     // Set system database before MFC initilizes DAO:
     AfxDaoSetSystemDB(strSystemDB);

     // User name and password manually added by using Microsoft Access:
     CString strUserName = _T("NewUser");
     CString strOldPassword = _T("Password");
     CString strNewPassword = _T("NewPassword");

     // Set default user so that MFC will be able to log in by default
     // using the user name and password from the system database:
     AfxDaoSetDefaultUser(strUserName,strOldPassword);

     // Change the password. You should be able to call this function from
     // anywhere in your MFC application:
     ChangePassword(strUserName,strOldPassword,strNewPassword);

   // Remainder of your InitInstance code:
   ...

   }

REFERENCES