Microsoft KB Archive/248400

= An ATL COM dynamic-link library that registers and creates a window may cause an access violation in Windows NT or in Windows 2000 =

Article ID: 248400

Article Last Modified on 9/2/2005

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APPLIES TO

 Microsoft ActiveX Template Library 2.0, when used with:  Microsoft Visual C++ 5.0 Enterprise Edition

 Microsoft Visual C++ 6.0 Enterprise Edition  Microsoft ActiveX Template Library 2.1, when used with:  Microsoft Visual C++ 5.0 Enterprise Edition

 Microsoft Visual C++ 6.0 Enterprise Edition</li></ul> </li> Microsoft ActiveX Template Library 3.0, when used with:  Microsoft Visual C++ 5.0 Enterprise Edition</li></ul>

 Microsoft Visual C++ 6.0 Enterprise Edition</li></ul> </li> Microsoft Windows NT Server 4.0 Standard Edition</li> Microsoft Windows NT Workstation 4.0 Developer Edition</li> Microsoft Windows 2000 Advanced Server</li> Microsoft Windows 2000 Server</li> Microsoft Windows 2000 Professional Edition</li></ul>

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This article was previously published under Q248400

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SYMPTOMS
An ATL Component Object Model (COM) dynamic-link library (DLL) that registers and creates a window and that runs on the Windows NT or Windows 2000 operating system may cause an access violation.

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CAUSE
ATL does not unregister the window classes that it registers.

On a Windows 9x-based computer, all window classes that are registered by a DLL are unregistered when the DLL is unloaded. However, in the Windows NT and Windows 2000 operating systems, no window classes registered by a DLL are unregistered when the DLL is unloaded.

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RESOLUTION
Unregister all of the window classes of the windows that you are creating inside of your ActiveX control. Use the code shown in the "Sample Code" section to unregister the window classes in the destructor of your class.

Resolution for CContainedWindow Use
If you have a CContainedWindow object in your class, the m_lpszClassName member of the CContainedWindow class stores the class name that was used to register the class. The sample code uses the same logic that is used by ATL (in Atlwin.cpp, CContainedWindow::RegisterWndSuperclass) to construct the window class name, and uses that name to unregister the class:

Sample Code LPTSTR szBuff = (LPTSTR)_alloca((lstrlen(m_MyContainedWnd.m_lpszClassName) + 14) * sizeof(TCHAR)); lstrcpy(szBuff, _T("ATL:")); lstrcat(szBuff, m_MyContainedWnd.m_lpszClassName); ::UnregisterClass(szBuff, _Module.GetModuleInstance); Replace m_MyContainedWnd with your CContainedWindow object name.

Resolution for CWindowImpl Use
If you have an ActiveX control or use one of the DECLARE_* macros, use code similar to the following: ::UnregisterClass(CMyClass::GetWndClassInfo.m_wc.lpszClassName, CMyClass::GetWndClassInfo.m_wc.hInstance); Replace CMyClass with your class name.

If you have multiple contained windows or have a contained window in a control, use both of these resolutions, according to how you have used the contained windows.

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STATUS
Microsoft has confirmed that this is a problem in the Microsoft products that are listed at the beginning of this article.

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MORE INFORMATION
For more information about ATL Control, click the following article number to view the article in the Microsoft Knowledge Base:

167526 FIX: ATL Control May Cause an Access Violation

Steps to Reproduce the Problem
This problem is difficult to reproduce. This behavior only occurs under the following conditions (for example):
 * 1) A client invokes two COM DLLs, DLL A and DLL B.
 * 2) Assume that DLL A registers a window class that is named "Button" and creates a window of that type, and DLL B registers the same window class named "Button" and creates another window.
 * 3) Invoke both the DLLs. This action registers the corresponding window classes.
 * 4) Force both of the DLLs to unload. You can use the ::CoFreeUnusedLibraries function to do this.

At this point the registered window classes are not unregistered in Windows NT and Windows 2000.
 * 1) If you load either one of the DLLs (A or B) again, and if they reload in exactly the same address of the other module (the address where the other one was loaded previously before getting unloaded), you will see a failure in the CreateWindowEx call for the Button window, which was registered in both the DLLs with the same class name.

Here is a brief explanation of why this problem happens:

The HINSTANCE value of a DLL is equal to the base address into which it was loaded. When DLL A registered the Button class, it assigned a WndProc address for the class in the address range of where it was loaded. DLL B also registered another class with the same name, but with a different WndProc location in its address space. Please note these window classes are local to the module that registered them. Thus, Button was registered from two different DLLs, even though the two DLLs were loaded in the same process. The HINSTANCE value determines whether a given DLL has already registered a particular window class.

When you create a window by using ATL, ATL firsts attempts to find whether a window class was registered for the given window by using the ::GetClassInfoEx function. If the class is not registered, then ATL will register the class and create the window. In the scenario in the "Steps to Reproduce the Problem" section, DLL A and DLL B both register the Button class with different WndProc addresses. Assuming DLL B loaded first the second time around, this time at the base address of DLL A, the ::GetClassInfoEx function returns the WndProc address that was registered by DLL A for the Button class. The address is invalid, and the first call to it causes an access violation.

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