PRB: When to Call AfxDaoTerm() in an Automation Server (152315)
The information in this article applies to:
- The Microsoft Foundation Classes (MFC), when used with:
- Microsoft Visual C++, 32-bit Editions 4.0
- Microsoft Visual C++, 32-bit Editions 4.1
- Microsoft Visual C++, 32-bit Enterprise Edition 4.2
- Microsoft Visual C++, 32-bit Professional Edition 4.2
- Microsoft Visual C++, 32-bit Enterprise Edition 4.2b
- Microsoft Visual C++, 32-bit Professional Edition 4.2b
- Microsoft Visual C++, 32-bit Enterprise Edition 5.0
- Microsoft Visual C++, 32-bit Professional Edition 5.0
This article was previously published under Q152315 SYMPTOMS
MFC DLLs and OCXs that use DAO need to call AfxDaoTerm() to terminate DAO
before their ExitInstance() function is called. MFC, by default, calls
AfxDaoTerm() from within CWinApp::ExitInstance().
An automation server may expose multiple objects that use DAO. Calling
AfxDaoTerm() when one of these objects is destructed may have the undesired
effect of terminating DAO for all other objects that may still be in use.
RESOLUTION
MFC keeps an internal object lock count of automation objects in the module
state. The constructor of each object exposed by the automation server
calls AfxOleLockApp() to increment this count and calls AfxOleUnlockApp()
to decrement it.
An MFC DLL automation server gets unloaded when the AfxOleCanExitApp()
function returns TRUE. This function checks the object lock count and
returns TRUE if no objects are in use. The default implementation of
DllCanUnloadNow() in an AppWizard-generated automation server looks like
this:
STDAPI DllCanUnloadNow(void)
{
AFX_MANAGE_STATE(AfxGetStaticModuleState());
return AfxDllCanUnloadNow();
}
Add the following code to terminate DAO when no automation objects are
in use any longer:
STDAPI DllCanUnloadNow(void)
{
AFX_MANAGE_STATE(AfxGetStaticModuleState());
if (AfxOleCanExitApp())
AfxDaoTerm();
return AfxDllCanUnloadNow();
}
STATUS
This behavior is by design.
REFERENCES
For more information, please refer to the following articles in the
Microsoft Knowledge Base:
143084 FIX: Problems with Using the MFC DAO Classes in a .DLL or .OCX
149889 PRB: Assertion or Problems Using DAO in a DLL
| Modification Type: | Major | Last Reviewed: | 12/10/2003 |
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| Keywords: | kbDatabase kbprb kbProgramming KB152315 |
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