Q123652: FIX: Update() Fails to Write Changed Text Field

Article: Q123652
Product(s): Microsoft C Compiler
Version(s): 2.00
Operating System(s): 
Keyword(s): kbDatabase kbMFC kbODBC kbVCkbbuglist kbfixlist
Last Modified: 29-JUL-2001

-------------------------------------------------------------------------------
The information in this article applies to:

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

SYMPTOMS
========

The CRecordset::Update() function fails to correctly update a text field if that
field is changed from a null value to a non-null value. The Update() function
may return FALSE or, if the dirty flag is set for the field, the Update()
function may return TRUE but a null value will be written to the record instead
of the expected non-null value.

For example, the following code worked with Visual C++ version 1.50 or 1.51, but
it may fail in Visual C++ version 2.0 given the conditions described above:

     recordset.Edit();
     recordset.Field1String="SomeText";
     recordset.Update();

CAUSE
=====

The RFX_Text() function, which maps a CString to a text field, has changed in
Visual C++ version 2.0. In Visual C++ version 1.51, you'll see the following
code in the MarkForUpdate operation of the RFX_Text() function:

    case CFieldExchange::MarkForUpdate:
          {
              if (value.IsEmpty())
                  pFX->m_prs->SetFieldFlags(nField,
                      AFX_SQL_FIELD_FLAG_NULL, pFX->m_nFieldType);
              else
                  pFX->m_prs->ClearFieldFlags(nField,
                      AFX_SQL_FIELD_FLAG_NULL, pFX->m_nFieldType);
              ...

This code was removed in Visual C++ version 2.0. The code checks to see if a
CString is empty and sets or resets the null flag accordingly. Without this
code, the null flag for a field will remain set even though you may change the
contents of a CString field variable. Because this code is no longer present in
the Visual C++ version 2.0 database classes code, you will have to set or reset
the null flag for a text field in your code.

RESOLUTION
==========

If you use a CRecordView for retrieving data, you won't notice the bug because a
CRecordView sets or resets a field's null flag depending on whether a user has
entered text into the edit control representing that field. The DDX_FieldText()
function does the work of setting the field's null flag.

For example, in the AfxFieldText() function, which gets called from the
DDX_FieldText() functions, you'll see this code:

  *****
  BOOL AFXAPI AfxFieldText(CDataExchange* pDX, int nIDC, void* pv,
      CRecordset* pRecordset)
  {
      ASSERT_VALID(pRecordset);

      HWND hWndCtrl = pDX->PrepareEditCtrl(nIDC);
      char szT[2];
      if (pDX->m_bSaveAndValidate)
      {
          ::GetWindowText(hWndCtrl, szT, sizeof(szT));
          if (szT[0] == '\0')
          {
              if (pRecordset->IsFieldNullable(pv))
              {
                  pRecordset->SetFieldNull(pv);
                  return TRUE;
              }
          }
          else
              pRecordset->SetFieldNull(pv, FALSE);
      }
      else
      {
          if (!pRecordset->IsOpen() || pRecordset->IsFieldNull(pv))
          {
              szT[0] = '\0';
              AfxSetWindowText(hWndCtrl, szT);
              return TRUE;
          }
      }
      return FALSE;
  }
  *****

You can see how the null flag for the field is set or reset. You can write
similar code to handle setting or resetting the null flag in your code.

As an alternative, you can create a new RFX function, and insert the code shown
above for the MarkForUpdate operation. Replace the call to RFX_Text() in your
CRecordset's DoFieldExchange() function with your new RFX function.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This bug was corrected in Visual C++ 2.1.

REFERENCES
==========

For more information about Record Field Exchange (RFX) functions see the
following references:

  MFC Technote #43 - "RFX Routines"
  MFC Encyclopedia articles on RFX in the online books

Additional query words: 2.00 3.00

======================================================================
Keywords          : kbDatabase kbMFC kbODBC kbVC kbbuglist kbfixlist
Technology        : kbAudDeveloper kbMFC
Version           : 2.00
Issue type        : kbbug
Solution Type     : kbfix

=============================================================================

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