FIX: Dynasets w/ CLongBinary Fields Throws Incorrect Exception

• The Microsoft Foundation Classes (MFC) included with: - Microsoft Visual C++, 32-bit Edition, version 4.0

SYMPTOMS

An MFC ODBC application that uses dynaset recordsets with CLongBinary bound fields worked in Visual C++ 2.x, but it throws an exception in Visual C++ 4.0 when the recordset is opened.

The CDBException indicates: Data truncated.

Additionally, the following trace messages are output (Microsoft Access driver output is shown):

   Error in row
   State:01S01,Native:89,Origin:[Microsoft][ODBC Microsoft Access 7.0
   Driver]

   Data truncated
   State:01004,Native:2,Origin:[Microsoft][ODBC Microsoft Access 7.0
   Driver]

   Error: field data truncated during Open or Requery.
   Data truncated.

CAUSE

The exception is thrown incorrectly. The problem is a bug in the MFC code. Both CRecordset::InitRecord() (responsible for initializing the recordset with the first record of the result set) and CRecordset::Move() perform a check following the fetch. If the fetch returns SQL_SUCCESS_WITH_INFO and the subsequent call to SQLError indicates that data was truncated, an exception should only be thrown if snapshots are being used. If the recordset is a dynaset, then data truncation is expected because of the way in which the MFC classes handle binding in dynasets, so it should be ignored.

The MFC code performs an incorrect comparison to determine if dynasets are being used resulting in the exact opposite of the desired effect -- exceptions are only thrown if dynasets are being used with CLongBinary fields. The incorrect code is shown below (from CRecordset::InitRecord()):

   if (!((m_pDatabase->m_dwUpdateOptions & AFX_SQL_POSITIONEDSQL) &&
            m_bLongBinaryColumns))
     {
        //throw the exception
     }

RESOLUTION

Unfortunately, CRecordset::InitRecord() is not a virtual function. Consequently, the two virtual functions that call InitRecord() must be overridden to allow a corrected version of InitRecord() to be called. Note that after you apply the following workaround, the trace messages will still be output but can be safely ignored.

NOTE: To assist you in implementing a workaround for this problem, obtain NOTRUNC.EXE:

The following file is available for download from the Microsoft Software Library:

 ~ Notrunc.exe (size: 27189 bytes) 

   ARTICLE-ID: Q119591
   TITLE     : How to Obtain Microsoft Support Files from Online Services

Workaround Example

If you downloaded Notrunc.exe, go to step 8. If you want to implement the modifications yourself, proceed with step 1:

1. Using ClassWizard, add a new class derived from CRecordset to your project (referred to as CNewRecordset hereafter) and specify the file name as Notrunc.cpp and Notrunc.h.

   a. Clear the Bind All Columns database option.

   b. Select a datasource and table at random.

2. Using ClassWizard, override the virtual functions Move() and Requery() for CNewRecordset:

   a. Remove the code provided by ClassWizard so that the bodies of

         the functions are empty.
   

   b. Copy the bodies of MFC's implementations of these functions into

         your overrides verbatim. These implementations can be found in
         Mfc\Src\Dbcore.cpp.
   

3. Add a function to CNewRecordset that will be your own implementation of InitRecord() (referred to as NewInitRecord() hereafter). Copy the MFC implementation of InitRecord() into your NewInitRecord() function - also from Dbcore.cpp.

4. Copy the declarations of both szRowFetch and szDataTruncated from Dbcore.cpp to Notrunc.cpp. Be sure to place the declarations above both the Move() override and your NewInitRecord() function.

5. Remove the four lines from Notrunc.cpp that contain the macro NO_CPP_EXCEPTION. This macro is only used when rebuilding the MFC libraries; it is unnecessary in this case.

6. Create an empty derivation from CDatabase by placing the following text at the top of Notrunc.h. (This class will be referred to as CnewDatabase hereafter.)

         class CNewDatabase : public CDatabase
         {
            friend class CNewRecordset;
         };
   
   

7. In Notrunc.cpp, perform the following text replacements:

   Replace 2 occurrences of:

         m_pDatabase->m_dwUpdateOptions
   

   With:

         ((CNewDatabase *)m_pDatabase)->m_dwUpdateOptions
   

   Replace 2 occurrences of:

         AFX_SQL_POSITIONEDSQL
   

   With:

         AFX_SQL_SETPOSUPDATES
   

   Replace 2 occurrences of:

         InitRecord
   

   With:

         NewInitRecord
   

8. Change the .h and .cpp files of your existing recordsets so that they derive from CNewRecordset.

   a. In the header files, change the class declarations:

         class CMySet : public CNewRecordset
   

   b. In the implementation files, change the constructor to call the

         CNewRecordset constructor:
   

         CMySet::CMySet(CDatabase* pdb) : CNewRecordset(pdb)
   

9. Include the Notrunc.h file in each recordset .h file that uses the new CNewRecordset class or add the Notrunc.h to the bottom of Stdafx.h.

10. Rebuild your project. Your application should no longer throw the undesired exception. Note that the trace messages will still be output, but these can be safely ignored.

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++ 4.1.