INFO: Desktop Drivers Not Meant For Access or DAO (154643)

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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.