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Q154643: INFO: Desktop Drivers Not Meant For Access or DAO

Article: Q154643
Product(s): Open Database Connectivity (ODBC)
Version(s): 3.40
Operating System(s): 
Keyword(s): 
Last Modified: 21-DEC-2000

3.40.2728

WINDOWS

kbnetwork

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The information in this article applies to:

- Microsoft Open Database Connectivity, version 3.40 
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SUMMARY
=======

The Microsoft ODBC Desktop Drivers should not be called from a Jet-based
application such as Access, Visual Basic bound data controls, Data Access
Objects (DAO) or the Visual C++ DAO classes.

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

The Microsoft Jet Database Engine stores its native databases in Access *.mdb
file format. If you load Installable ISAM DLL files, Jet can also work directly
with dBASE, Microsoft Excel, Microsoft FoxPro, Paradox, and Text files. This
means that the three Microsoft products which have been developed using the Jet
Database Engine have native access to all of these file types. In addition to
working directly with these file types, Jet can work against any data source
which provides an ODBC driver. This is called attaching an external ODBC data
source or table.

The first Microsoft product developed using Jet is Access. All database work done
in Access goes through Access's instance of Jet, either against its native *.mdb
files or the other file types it can access directly using Installable ISAM's.
The second product developed using Jet is the Data Access Objects (DAO)
implemented in both Visual Basic and the Visual C++ DAO classes. DAO can also
work natively against Access *.mdb files and the files accessed through the
Installable ISAM's. Both Access and DAO can also work against any ODBC driver to
work with file types or databases not covered by Jet's Installable ISAM's. The
third Microsoft product group developed using the Jet engine is the Microsoft
Desktop Drivers for Microsoft Access, dBASE, Microsoft Excel, Microsoft FoxPro,
Text, and Paradox. The Desktop Drivers are different than Access and DAO in that
each driver loads a specific Installable ISAM DLL file; therefore, they are
limited to working with the file type covered by that Installable ISAM.

When an Access or DAO application is run, it starts an instance of the Jet engine
for the application process. If this application then attempted to connect to an
ODBC Desktop Driver, the process would have to load a second instance of Jet to
implement the driver. Jet is not architected in a way that will allow two
instances of Jet to be run for a single process, so this results in various
errors. Applications using Access or DAO must use their native mechanisms to
work with .mdb files or the files accessed through Installable ISAM's. The
Desktop Drivers are provided for use from applications utilizing API's not based
on the Jet engine. Examples where the Desktop Drivers can be used would be
Visual Basic Remote Data Objects (RDO), Visual C++ MFC Database Classes, or
applications written directly to the ODBC API.

Jet-based applications cannot use the Desktop Drivers even with SQL_PASSTHROUGH.
SQL_PASSTHROUGH is an option which Access or DAO programmers can use to instruct
Jet not to process a SQL command and simply pass it directly to an attached ODBC
driver. The Access or DAO application must load its instance of Jet however, and
this may cause problems for the Jet instance of a Desktop Driver. Visual Basic
or Visual C++ applications cannot have any DAO code at all if they are going to
work with Desktop Drivers. In one support case, a Visual Basic application was
converted from DAO to the ODBC API, but it was still getting errors when it
attempted to use the Access ODBC driver. The cause was found to be that one DAO
line, a dim database, had been left in the application. Even though this dim
database was never actively referenced in the code, it was sufficient to load a
DAO instance of Jet that caused problems for the Access driver's instance of
Jet. The application worked correctly when the dim database was removed.

Not all Microsoft drivers are implemented using the Jet Database Engine, and
these issues do not apply to them. Examples are the Microsoft SQL Server and
Visual Foxpro ODBC drivers. Users can find out if a driver is based on Jet using
the ODBC Administrator. In the ODBC Administrator, click Drivers. In the list of
drivers, double-click the driver in question. If the driver's file name is
ODBCJT32.DLL or ODBCJT16.DLL, it is a Jet- based driver. If the file name is
something else, it is not a Jet-based driver and is not affected by the issues
discussed in this article.

Additional query words: odbc 3.40.2728

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Keywords          :  
Technology        : kbAudDeveloper kbODBCSearch
Version           : :3.40
Issue type        : kbinfo

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