When to Respond to the WM_GETOBJECT Message

If an application supports Microsoft Active Accessibility or UI Automation for a UI element, the application must not respond to the WM_GETOBJECT message before the object that represents the UI element is fully initialized, or after the application has begun to close. When an application creates a new window, the system raises the EVENT_OBJECT_CREATE WinEvent to notify clients before sending the WM_CREATE message to the application's window procedure. Because many applications use WM_CREATE to start the initialization process, any accessibility implementation of a UI element does not respond to the WM_GETOBJECT message until after the application has finished processing the WM_CREATE message.