Knowledge Base

PRB: DAO 3.0 Must Be Used in Primary Thread

Article ID: 151407

Article Last Modified on 11/21/2006

Microsoft Foundation Class Library 4.2, when used with:
- Microsoft Visual C++ 4.0 Standard Edition
- Microsoft Visual C++ 4.1 Subscription
- Microsoft Visual C++ 4.2 Enterprise Edition
- Microsoft Visual C++ 4.2 Professional Edition

This article was previously published under Q151407

SYMPTOMS

Attempting to use either the MFC DAO classes or the DAO SDK dbDao classes in any thread but the primary one causes unexpected behavior.

CAUSE

DAO 3.0 is not marked as apartment-model or free-threaded in the registry. No multithreading testing has been done on DAO 3.0. Any documentation should say that DAO 3.0 is not thread-safe and should only be used in the primary thread in a process.

RESOLUTION

Do not use MFC DAO or the DAO SDK dbDao classes in any thread but the primary thread of an application.

STATUS

This behavior is by design.

MORE INFORMATION

Internally, the Jet engine is multithreaded, and the registry setting \HKEY_LOCALMACHINE\Software\Microsoft\Jet\3.0\Engines\Jet\Threads controls the number of threads that the engine uses internally. The Jet engine is multithreaded internally, but neither DAO nor the Jet engine are thread- safe.

For more information on OLE threading models, see the following article in the Microsoft Knowledge Base:

150777 Descriptions and Workings of OLE Threading Models

Tech Note #64: Apartment-Model Threading in OLE Controls

REFERENCES

147629 PRB: Problems if ODBC or DAO in DLL's InitInstance or DLLMain

Keywords: kbdatabase kbprb kbprogramming kbusage KB151407