PRB: MFC DB Classes Don't Update NULL Field Values to Non-NULL

ID: Q130956

2.00 WINDOWS NT kbprb

The information in this article applies to:

The Microsoft Foundation Classes (MFC) included with: Microsoft Visual C++, 32-bit Edition, version 2.0

SYMPTOMS

When the MFC Database classes that come with Visual C++ version 2.0 are used, any field that has a NULL value cannot be updated to a non-NULL value successfully.

After opening a recordset on a SQL Server table that has fields with NULL values, the Edit() and Update() methods of the recordset fail to update the value of the field to a non-NULL value. For example, the following code isn't able to do the update:

   CMyRecordSet rs;
   rs.Open();
   rs.Edit();
   rs.m_Name="Test"; //m_Name allows NULLs and is NULL for current record
   rs.Update();

The Update() function doesn't return any error, but the m_Name value is still set to NULL.

RESOLUTION

Use the MFC Database Classes that come with Visual C++ version 2.1 or above. Otherwise, for the field to be updated, force the bNull flag of the SetFieldNull() method to FALSE before calling the Update(). The following code works:

   CMyRecordSet rs;
   rs.Open();
   rs.Edit();
   rs.m_Name="Test";  //m_Name allows NULLs and is NULL for current record
   rs.SetFieldNull(&rs.m_Name, FALSE);
   rs.Update();

STATUS

This behavior is by design.

Additional reference words: 2.00 3.00 ODBC KBCategory: kbprb KBSubcategory: MfcDatabase

Keywords          : kbcode kbDatabase kbMFC kbODBC kbVC 
Version           : 2.00
Platform          : NT WINDOWS

Last Reviewed: May 22, 1998