FIX: Setting Default Value Using CreateField Gives Wrong Value (171316)

CAUSE

There is a bug in version 4.21.7160 of MFC42.DLL (this is the MFC DLL that comes with Visual C++ 5.0 Service Pack 1). In the AfxSetDefaultValue() function located in DAOCORE.CPP, you can see the following code:

   .
   .
   .
   else
   {
      // Call DAO 3.0 method
      // put_DefaultValue takes BSTR param not VARIANT
      HRESULT (STDMETHODCALLTYPE DAOField::*pMethod)(BSTR) =
      (HRESULT (STDMETHODCALLTYPE
       DAOField::*)(BSTR))pDAOField->put_DefaultValue;
      BSTR bstr = strDefaultValue.AllocSysString();
      DAO_CHECK((pDAOField->*pMethod)(bstr));
      SysFreeString(bstr);
   }

The bstr value needs to be created as an ANSI BSTR, which DAO expects in a non-UNICODE application rather than a UNICODE BSTR that the code is creating.

RESOLUTION

You can work around this problem by deriving a new class from CDaoTableDef that defines a new CreateField() function. If you use this code in your Visual C++ 4.2x application and later decide to build your application with Visual C++ 5.0 or greater (which uses DAO 3.5 and doesn't experience the bug), you don't want to use new class. The code demonstrates one way that you can check the _MFC_VER constant to determine whether the code is needed:

.H File

   #if _MFC_VER <0x0421
   class CNewDaoTableDef: public CDaoTableDef
   {
   public:
      CNewDaoTableDef(CDaoDatabase * pDatabase):CDaoTableDef(pDatabase){};

      void CreateField(CDaoFieldInfo& fieldinfo);
      void SetFieldInfo(DAOField* pDAOField, CDaoFieldInfo& fieldinfo);
   };
   #endif  // _MFC_VER <0x421

.CPP File

   #if _MFC_VER <0x0421
   void CNewDaoTableDef::CreateField(CDaoFieldInfo& fieldinfo)
   {
      ASSERT_VALID(this);

      DAOField* pDAOField;

      // Create the DAO field object (setting basic properties)
      DAO_CHECK(m_pDAOTableDef->CreateField(
                COleVariant(fieldinfo.m_strName, VT_BSTRT),
                COleVariant(fieldinfo.m_nType),
                COleVariant(fieldinfo.m_lSize), &pDAOField));
      TRY
      {
         SetFieldInfo(pDAOField, fieldinfo);

         // Append the field object to the fields collection
         if (m_pDAOFields == NULL)
            InitFieldsCollection();

         DAO_CHECK(m_pDAOFields->Append(pDAOField));
       }
       CATCH_ALL(e)
       {
         pDAOField->Release();
         THROW_LAST();
       }
       END_CATCH_ALL

       pDAOField->Release();
   }

   void CNewDaoTableDef::SetFieldInfo(DAOField* pDAOField, CDaoFieldInfo&
                                      fieldinfo)
   {
      // Assumes name, type and size set on direct DAO CreateField call
      ASSERT(pDAOField != NULL);

      if (fieldinfo.m_lAttributes != 0)
        DAO_CHECK(pDAOField->put_Attributes(fieldinfo.m_lAttributes));

      if (fieldinfo.m_nOrdinalPosition != 0)
      {
      DAO_CHECK(pDAOField
                ->put_OrdinalPosition(fieldinfo.m_nOrdinalPosition));
      }

      if (fieldinfo.m_bRequired)
         DAO_CHECK(pDAOField->put_Required(AFX_DAO_TRUE));

      if (fieldinfo.m_bAllowZeroLength)
         DAO_CHECK(pDAOField->put_AllowZeroLength(AFX_DAO_TRUE));

      if (!fieldinfo.m_strForeignName.IsEmpty())
      {
         COleVariant var(fieldinfo.m_strForeignName, VT_BSTRT);
         DAO_CHECK(pDAOField->put_ForeignName(V_BSTR(&var)));
      }

      if (!fieldinfo.m_strValidationRule.IsEmpty())
      {
        COleVariant var(fieldinfo.m_strValidationRule, VT_BSTRT);
        DAO_CHECK(pDAOField->put_ValidationRule(V_BSTR(&var)));
      }

      if (!fieldinfo.m_strValidationText.IsEmpty())
      {
        COleVariant var(fieldinfo.m_strValidationText, VT_BSTRT);
        DAO_CHECK(pDAOField->put_ValidationText(V_BSTR(&var)));
      }

      if (!fieldinfo.m_strDefaultValue.IsEmpty())
      {
        COleVariant var(fieldinfo.m_strDefaultValue, VT_BSTRT);
        DAO_CHECK(pDAOField->put_DefaultValue(V_BSTR(&var)));
      }
   }
   #endif // _MFC_VER <0x0421

MORE INFORMATION

Sample Code

This sample code fails if the application is built with Visual C++ 4.2x and you use the newer MFC42.DLL (the one with Visual C++ 5.0 service pack 1).

   CDaoDatabase db;
   db.Open(_T("db.mdb"));

   CDaoTableDef td(&db);
   // Replace the line above with the following code to fix the problem:
   // #if _MFC_VER < 0x0421
   //   CNewDaoTableDef td(&db);  // need to use new class w/ VC++ 4.2x
   // #else
   //   CDaoTableDef td(&db); // building with VC++ 5.0 the bug doesn't
   // exist #endif

   td.Create(_T("FieldTest"));

   CDaoFieldInfo fi;
   fi.m_strName = _T("testfield");
   fi.m_nType = dbText;
   fi.m_lSize = 50;
   fi.m_lAttributes = dbFixedField;
   fi.m_nOrdinalPosition = 0;
   fi.m_bRequired = FALSE;
   fi.m_bAllowZeroLength = TRUE;
   fi.m_lCollatingOrder = dbSortGeneral ;
   fi.m_strDefaultValue = _T("default value");
   td.CreateField(fi);
   td.Append();

   CDaoRecordset rs(&db);
   rs.Open(dbOpenDynaset, _T("Select * from FieldTest3"));
   rs.AddNew();
   rs.Update();
   rs.Close();
   db.Close();