FIX: CDaoTableDef CreateField Cannot Set "Allow Zero Length"

ID: Q148805

The information in this article applies to:

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));

Because the default in Microsoft Access is FALSE (does not allow zero length), the above code doesn't set it to TRUE.

RESOLUTION

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 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.

Additional query words:

Keywords : kbprg kbDAO kbDatabase kbMFC kbVC
Version : winnt:
Platform : winnt
Issue type : kbbug