FIX: Setting Default Value Using CreateField Gives Wrong Value

• Microsoft Visual C++, 32-bit Editions, version 5.0sp1

SYMPTOMS

An MFC DAO application built using Visual C++ version 4.2x and linked to MFC42.DLL incorrectly sets the default value for a field. For example, if CDaoTableDef::CreateField() is called with the CDaoFieldInfo::m_strDefaultValue set to "default value", you will find that a "d" is added as the default value instead of the whole string.

For sample code that demonstrates the problem, see the MORE INFORMATION section below.

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);
   }

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

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug has been fixed in Visual Studio 97 Service Pack 3.

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

   ARTICLE-ID: Q170365
   TITLE     : INFO: Visual Studio 97 Service Packs - What, Where, and Why

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();