FIX: CDaoTableDef CreateField Cannot Set "Allow Zero Length"

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

SYMPTOMS

The CDaoFieldInfo structure has a member variable, m_bAllowZeroLength, that allows you to set the "Allow Zero Length" property for a text or memo field in a Microsoft Access table. However, m_bAllowZeroLength cannot be set to TRUE in an MFC DAO application. The application does not complain when you call CDaoTableDef::CreateField () to set this to TRUE, but the table in Microsoft Access shows that the field property does not allow zero length. Later if you try to put a NULL value in the field, a CDAOException is thrown which says "Field cannot be Null."

CAUSE

AfxSetFieldInfo in Daocore.cpp (which is called by CreateField) has this code:

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

RESOLUTION

1. Copy the code for AfxSetFieldInfo() from Daocore.cpp into your project as a separate function. Then modify the line containing the bug. Remove the exclamation mark (!) in the first line, and change AFX_DAO_FALSE to AFX_DAO_TRUE in the second line:

      void AFX_CDECL MyAfxSetFieldInfo(DAOField* pDAOField,
                           CDaoFieldInfo&  fieldinfo)
      {
      ...
      // Here you fix the bug
   
      if (fieldinfo.m_bAllowZeroLength)
         DAO_CHECK(pDAOField->put_AllowZeroLength(AFX_DAO_TRUE));
      ...
      }
   
   

2. Instead of calling CDaoTableDef.CreateField(), write your own function to call the DAO function CreateField() directly by using the tabledef's m_daoTableDef pointer. The following example shows how to do that:

      void MyCreateField()
      {
      CDaoDatabase db;
      db.Open("c:\\msdev\\projects\\daoenrol\\stdreg32.mdb",
         FALSE, FALSE);
   
      // create a tabledef
      CDaoTableDef td ( &db );
      td.Create(_T("Table1"),0,_T("Table1"), NULL);
   
      // create a FieldInfo structure
      struct CDaoFieldInfo m_FI;
   
      m_FI.m_nOrdinalPosition = 0;
   
      // always initialize the rest
      m_FI.m_strName = _T("MyField");
      m_FI.m_nType = dbText ;
      m_FI.m_lSize = 20;
      m_FI.m_bAllowZeroLength = TRUE;  // To set "Allow Zero Length"
   
      m_FI.m_strDefaultValue = _T("");
      m_FI.m_bRequired = FALSE;
      m_FI.m_lAttributes = dbUpdatableField;
   
      m_FI.m_strValidationRule = _T("");
      m_FI.m_strValidationText = _T("");
   
      // Copy code from CDaoTableDef::CreateField()
   
      DAOField* pDAOField;
   
      // Create the DAO field object (setting basic properties)
   
      // DAO_CHECK(m_pDAOTableDef->CreateField(
      DAO_CHECK(td.m_pDAOTableDef->CreateField(
         COleVariant(m_FI.m_strName, VT_BSTRT),
         COleVariant(m_FI.m_nType),
         COleVariant(m_FI.m_lSize), &pDAOField));
   
      TRY
      {
      // Basic properties already set, so set the rest
      MyAfxSetFieldInfo(pDAOField, m_FI);
   
      // Append the field object to the fields collection
      if (td.m_pDAOFields == NULL)
          // cannot call private member function
          //td.InitFieldsCollection();
          DAO_CHECK(td.m_pDAOTableDef->get_Fields (&(td.m_pDAOFields)));
   
      DAO_CHECK(td.m_pDAOFields->Append(pDAOField));
      }
      CATCH_ALL(e)
      {
         pDAOField->Release();
         THROW_LAST();
      }
      END_CATCH_ALL
   
      pDAOField->Release();
   
      td.Append();
      }
   
   

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This problem was corrected in Microsoft Visual C++, 32-bit Edition, version 4.1.

Keywords          : MfcDAO kbprg kbbuglist kbfixlist
Technology        : kbMfc
Version           : 4.0
Platform          : NT WINDOWS
Issue type        : kbbug
Solution Type     : kbfix