PRB: Cannot Exit Windows with DLL-Instanced Windows Open

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.1 and 3.0

SYMPTOMS

When the user closes the Windows Program Manager and chooses the OK button in the Exit Windows dialog box, Windows does not close.

CAUSE

A window was created in a window class registered by a dynamic-link library (DLL), and the DLL was unloaded from the system before the window was destroyed.

RESOLUTION

Before the DLL is unloaded from the system, explicitly destroy all windows created in a window class registered by the DLL.

MORE INFORMATION

When a DLL is unloaded from memory, all window classes registered by that DLL are purged. Any windows that have been created with a DLL- registered window class become invalid when the window class is purged; however, these windows are not destroyed.

Sending any message to an invalid window returns zero. When Windows sends a WM_QUERYENDSESSION message to an invalid window, the window returns zero and Windows does not shut down. Under the debugging version of Windows, this action creates Fatal Exit code 0x0007 messages.

This problem does not occur under Windows 3.1. When a DLL is unloaded, before its window classes are purged, any windows created using a DLL- registered window class are destroyed. When the debugging version of Windows 3.1 destroys a window in this manner, it produces one of the following four messages:

• Module unloaded with windows still subclassed
• Global class freed with existent class windows!
• Window class freed with existent class windows!
• USER: Window not destroyed