Q148805: FIX: CDaoTableDef CreateField Cannot Set "Allow Zero Length"

Article: Q148805
Product(s): Microsoft C Compiler
Version(s): winnt:
Operating System(s): 
Keyword(s): kbProgramming kbDAOsearch kbDatabase kbMFC kbVCkbbuglist kbfixlist
Last Modified: 03-AUG-2001

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), included with:
   - *EDITOR Please do not choose this product*Microsoft Visual C++ 32-bit Edition* use 241, 265, 225, 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
==========

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 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          : kbProgramming kbDAOsearch kbDatabase kbMFC kbVC kbbuglist kbfixlist
Technology        : kbAudDeveloper kbMFC
Version           : winnt:
Issue type        : kbbug
Solution Type     : kbfix

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