Q166756: FIX: Error in Requery When Set Null Date Parameter to Not Null

Article: Q166756
Product(s): Microsoft C Compiler
Version(s): 5.0,6.0
Operating System(s): 
Keyword(s): kbVC500bug kbVC600bug kbDSupportkbbuglist
Last Modified: 11-FEB-2002

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
   - Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
   - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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SYMPTOMS
========

When you set a previously Null date parameter to no longer be Null, using either
CRecordset::SetParamNull() or CRecordset::SetFieldNull(), it may cause a
subsequent call to CRecordset::Requery() to generate the following exception:

  Invalid string or buffer length

CAUSE
=====

During a call to CRecordset::Open, no memory is allocated for a Null date field.
However, CRecordset::Requery expects the memory to exist.

RESOLUTION
==========

Create a replacement for the RFX_Date function which always allocates memory for
the date field. See the MORE INFORMATION section.

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++ .NET.

MORE INFORMATION
================

This problem only occurs with the version of RFX_Date that takes a CTime
argument.

Steps to Reproduce Behavior
---------------------------

The following code displays the problem for a predefined query named Query1,
which takes one date parameter:

      CMySet rs;
      try
      {
          rs.SetParamNull( 0, TRUE );
          rs.Open( CRecordset::snapshot, "{Call Query1(?)}" );
          rs.m_paramDate = CTime( 1996, 10, 13, 0, 0, 0 );
          rs.SetParamNull( 0, FALSE );
          rs.Requery();
      }
      catch( CDBException* e )
      {
          AfxMessageBox( e->m_strError );
          e->Delete();
      }
      rs.Close();

Steps to Replace the RFX_Date Function
--------------------------------------

The following steps detail one method of making the needed changes in RFX_Date:

1. Copy the implementation of RFX_Date() (DEVSTUDIO\VC\MFC\SRC\DBRFX.CPP) into a
  new .cpp file, and rename the function to something such as RFX_Date2().
  Create a header file for the .cpp file and include that in your recordset
  .cpp file. Be sure to type:

        #include "stdafx.h"

  at the top of your new .cpp file, and include the .cpp file in your project.

2. Change all RFX_Date() calls for date parameters in your recordset's
  DoFieldExchange() to calls to RFX_Date2().

3. In your new RFX_Date2() implementation, comment out the following lines,
  which appear under the CFieldExchange::BindParam case:

         if (pFX->m_prs->IsParamStatusNull(nField - 1))
         {
             pts = NULL;
             *plLength = SQL_NULL_DATA;
         }
         else

4. To reduce the amount of code, you may want to remove all of the cases from
  the select statement except the case for CFieldExchange::BindParam, and add
  the following lines to the top of your replacement function:

         if( CFieldExchange::BindParam != pFX->m_nOperation ) {
             RFX_Date(pFX, szName, value);
             return;
         }

Additional query words: kbVC500bug kbVC600bug kbmfc kbdatabase kbodbc

======================================================================
Keywords          : kbVC500bug kbVC600bug kbDSupport kbbuglist
Technology        : kbAudDeveloper kbMFC
Version           : :5.0,6.0
Issue type        : kbbug
Solution Type     : kbfix

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