BUG: Cannot Set Access Memo Field to NULL with Snapshots

The information in this article applies to:

• Microsoft Visual C++ for Windows, versions 1.5, 1.51, and 1.52c

SYMPTOMS

When using a Snapshot recordset with the CRecordset class, if you delete the contents of a CString mapped to a memo field in a Microsoft Access database, nothing is actually deleted in the database. In other words, the data that previously existed in the field remains in that field. However, the recordset still shows that the field has been changed to null but it has only been changed in the cache of the cursor library, not the database.

CAUSE

A problem exists in the ODBC cursor library which is used by default in MFC.

RESOLUTION

This problem was resolved with the ODBC Cursor Library that ships with Visual C++ 4.0 for 32-bit ODBC components and remains unresolved for 16-bit ODBC components.

If possible, use dynasets in your application. For additional information on using dynasets with the 16-bit MFC database classes, please see the following article(s) in the Microsoft Knowledge Base:

   ARTICLE-ID: Q124915
   TITLE:      SAMPLE: Using Dynasets with the 16-Bit MFC Database Classes

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. We are researching this bug and will post new information here in the Microsoft Knowledge Base as it becomes available.

MORE INFORMATION

Sample Code

The code below is from a CRecordView derived class using a recordset for an Access data source, which has a memo field that maps to m_Memo.

    void CMyView::DoDataExchange(CDataExchange* pDX)
    {
        CRecordView::DoDataExchange(pDX);

        // Data Members --> Dialog
        if (pDX->m_bSaveAndValidate == 0)
        {
            if( m_pSet->m_Memo.GetLength() == 0 )
            {
                m_pSet->m_Memo = " ";
            }
        }

        //{{AFX_DATA_MAP(CMyView)
        ...
        //}}AFX_DATA_MAP

        // Dialog --> Data Members
        if (pDX->m_bSaveAndValidate != 0)
        {
            if( m_pSet->m_Memo.GetLength() == 0 )
            {
               m_pSet->m_Memo = " ";
            }
        }
    }