Q141303: Dynasets w/ CLongBinary Fields Throws Incorrect Exception
Article: Q141303
Product(s): Microsoft C Compiler
Version(s): winnt:4.0
Operating System(s):
Keyword(s): kbcode kbfile kbDatabase kbMFC kbODBC kbVC kbVC410fix
Last Modified: 06-MAY-2001
-------------------------------------------------------------------------------
The information in this article applies to:
- The Microsoft Foundation Classes (MFC), used with:
- Microsoft Visual C++, 32-bit Editions, version 4.0
-------------------------------------------------------------------------------
SUMMARY
=======
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.
The exception is thrown from CRecordset::InitRecord(). Also, in release builds,
the same exception is also thrown by CRecordset::Move(). 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
}
AFX_SQL_POSITIONEDSQL corresponds to the update option for a snapshot, not a
dynaset. 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 files are available for download from the Microsoft Download
Center:
Notrunc.exe
For additional information about how to download Microsoft Support files, click
the article number below to view the article in the Microsoft Knowledge Base:
Q119591 How to Obtain Microsoft Support Files from Online Services
Microsoft used the most current virus detection software available on the date of
posting to scan this file for viruses. Once posted, the file is housed on secure
servers that prevent any unauthorized changes to the file.
After running the Notrunc.exe, you will have a custom component file
(Notrunc.ogx), .cpp file, .h file, and a Readme.txt file. You can import the
.ogx file into the component gallery, and then reuse it as needed. For more
information on importing components, see Importing Components in the Visual C++
User's Guide.
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.
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.
MORE INFORMATION
================
Additional query words: kbVC400bug truncation long binary column
======================================================================
Keywords : kbcode kbfile kbDatabase kbMFC kbODBC kbVC kbVC410fix
Technology : kbAudDeveloper kbMFC
Version : winnt:4.0
Solution Type : kbfix
=============================================================================
THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.
Copyright Microsoft Corporation 1986-2002.