FIX: ATL Control May Cause an Access Violation

• Microsoft Active Template Library, versions 2.0, 2.1

SYMPTOMS

An ATL that superclasses a Windows control and runs on Windows NT can cause an access violation.

CAUSE

When ATL registers the Window Class for the control, it copies the WNDCLASSEX structure from the control it is superclassing. Because Windows controls are a global class, that attribute is given to the ATL control.

The problem occurs when the control’s DLL is unloaded and then reloaded in the same process. ATL tries to register the Window Class with the current WndProcAddress, but Windows NT does not allow a Global Class to be overwritten. If the DLL was reloaded at another address, the next call to the WndProc causes an access violation.

RESOLUTION

In the file Atlwin.cpp, insert the following line (on line 610)) in the function CContainedWindow::RegisterWndSuperclass() just before the call to RegisterClassEx :

   wc.style &= ~CS_GLOBALCLASS;  // don't register as global class
   atom = ::RegisterClassEx(&wc);

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug has been corrected in Visual Studio 97 Service Pack 1.

For additional information about the Visual Studio 97 Service Pack 1, please see the following article in the Microsoft Knowledge Base:

   ARTICLE-ID: Q170365
   TITLE     : INFO: Visual Studio 97 Service Packs - What, Where, and Why

MORE INFORMATION

This bug is hard to reproduce because it will only appear if the client application unloads and then reloads the DLL and then the DLL is loaded at another address. But any superclassed control built without the above resolution may fail under the right conditions.

(c) Microsoft Corporation 1997, All Rights Reserved. Contributions by Chuck Bell, Microsoft Corporation