FIX: CreateRelation Method Fails with Non-UNICODE Build

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

SYMPTOMS

The CDaoDatabase::CreateRelation() method may fail if an application is built as a non-UNICODE application. It is possible that a messagebox will appear that states:

   Invalid field name '<some text>' in definition of index or relationship

CAUSE

A bug exists in the CDaoDatabase::CreateRelation() function. In \Msdev\Mfc\Src\Daocore.cpp, you can see the following code in the CreateRelation() function:

   COleVariant var;
   var = relinfo.m_pFieldInfos[nIndex].m_strForeignName;
   DAO_CHECK(pDAOField->put_ForeignName(V_BSTR(&var)));

   COleVariant var(relinfo.m_pFieldInfos[nIndex].m_strForeignName,
                  VT_BSTRT);
   DAO_CHECK(pDAOField->put_ForeignName(V_BSTR(&var)));

RESOLUTION

Perform the following steps:

1. Derive a new class from CDaoDatabase.

2. Add the function prototypes for the two CreateRelation() functions to your new class declaration.

3. Copy the two CreateRelation() functions from Daocore.cpp to the CreateRelation() functions of your new CDaoDatabase-derived class.

4. Change the following code in your new CreateRelation(CDaoRelationInfo& relinfo) method from:

         COleVariant var;
         var = relinfo.m_pFieldInfos[nIndex].m_strForeignName;
         DAO_CHECK(pDAOField->put_ForeignName(V_BSTR(&var)));
   

   to:

         COleVariant var(relinfo.m_pFieldInfos[nIndex].m_strForeignName,
                         VT_BSTRT);
         DAO_CHECK(pDAOField->put_ForeignName(V_BSTR(&var)));
   
   

5. Declare a local VARIANT and initialize it in the overridden function as follows:

   VARIANT var; var.vt = VT_ERROR; var.scode = DISP_E_PARAMNOTFOUND;

   This is necessary because the _afxOptionalVariant used in the original CreateRelation() function is defined in DAOCORE.CPP,

6. Replace instances of the _afxOptionalVariant variable with var.

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.

MORE INFORMATION

Sample Code to Demonstrate Fixed Code

This demonstrates how you will use your new CDaoDatabase-derived class; in this case called CMyDatabase.

   CMyDatabase db;
   db.Open("f:\\test\\relation\\relate.mdb");
   db.CreateRelation(_T("foo"), _T("Table1"), _T("Table2"),
            dbRelationUnique|dbRelationUpdateCascade,_T("field1"),
            _T("field1"));
   db.Close();

   class CMyDatabase:public CDaoDatabase
   {
   public:
     void CreateRelation(LPCTSTR lpszName, LPCTSTR lpszTable,
                   LPCTSTR lpszForeignTable, long lAttributes,
                   LPCTSTR lpszField,
                   LPCTSTR lpszForeignField);
     void CreateRelation(CDaoRelationInfo& relinfo);
   };

   void CMyDatabase::CreateRelation(LPCTSTR lpszName, LPCTSTR lpszTable,
        LPCTSTR lpszForeignTable, long lAttributes, LPCTSTR lpszField,
        LPCTSTR lpszForeignField)
   {
     ASSERT_VALID(this);

     CDaoRelationInfo relinfo;
     CDaoRelationFieldInfo fieldinfo;

     relinfo.m_strName = lpszName;
     relinfo.m_strTable = lpszTable;
     relinfo.m_strForeignTable = lpszForeignTable;
     relinfo.m_lAttributes = lAttributes;
     relinfo.m_nFields = 1;

     relinfo.m_pFieldInfos = &fieldinfo;
     relinfo.m_pFieldInfos->m_strName = lpszField;
     relinfo.m_pFieldInfos->m_strForeignName = lpszForeignField;

     CreateRelation(relinfo);
   }

   void CMyDatabase::CreateRelation(CDaoRelationInfo& relinfo)
   {
     ASSERT_VALID(this);
     ASSERT(IsOpen());
     ASSERT(relinfo.m_nFields > 0);


     VARIANT var;
     var.vt    = VT_ERROR;
     var.scode = DISP_E_PARAMNOTFOUND;

     // Initialize relations collection so that relation can be
     // appended later
     if (m_pDAORelations == NULL)
         InitRelationsCollection();

     DAORelation* pDAORelation = NULL;
     DAOFields* pDAOFields = NULL;
     DAOField* pDAOField = NULL;

     // Create the relation
     DAO_CHECK(m_pDAODatabase->CreateRelation(
               COleVariant(relinfo.m_strName, VT_BSTRT),
               COleVariant(relinfo.m_strTable, VT_BSTRT),
               COleVariant(relinfo.m_strForeignTable, VT_BSTRT),
               COleVariant(relinfo.m_lAttributes), &pDAORelation));

     TRY
     {
        // Get the fields collection for later append of created field
        DAO_CHECK(pDAORelation->get_Fields(&pDAOFields));

        // Create field(s) and set the name and foreign name
        for (int nIndex = 0; nIndex < relinfo.m_nFields; nIndex++)
        {
            // note that we are using 'var' in the following function
            // instead of _afxOptionalVar declared in DAOCORE.CPP
            DAO_CHECK(pDAORelation->CreateField(
                    COleVariant(relinfo.m_pFieldInfos[nIndex].m_strName,
                                VT_BSTRT), var, var, &pDAOField));

           COleVariant var(relinfo.m_pFieldInfos[nIndex].m_strForeignName,
                           VT_BSTRT);
           DAO_CHECK(pDAOField->put_ForeignName(V_BSTR(&var)));

           // Append the field to relation fields collection and
           // release
           DAO_CHECK(pDAOFields->Append(pDAOField));
                     pDAOField->Release();
        }

        DAO_CHECK(m_pDAORelations->Append(pDAORelation));
     }
     CATCH_ALL(e)
     {
          // Clean up before throw
     if (pDAOField != NULL)
              pDAOField->Release();

          if (pDAOFields != NULL)
             pDAOFields->Release();

          pDAORelation->Release();
          THROW_LAST();
    }
    END_CATCH_ALL

    // Clean up
    if (pDAOField != NULL)
       pDAOField->Release();

    pDAOFields->Release();
    pDAORelation->Release();
   }

Keywords          : MfcDAO vcbuglist400 vcfixlist410 kbcode kbprg
Technology        : kbMfc
Version           : 4.0
Platform          : NT WINDOWS
Issue type        : kbbug
Solution Type     : kbfix