In general, an application must destroy all the windows it creates. It does this by using the DestroyWindow function. When a window is destroyed, the system hides the window, if it is visible, and then removes any internal data associated with the window. This invalidates the window handle, which can no longer be used by the application.
An application destroys many of the windows it creates soon after creating them. For example, an application usually destroys a dialog box window as soon as the application has sufficient input from the user to continue its task. An application eventually destroys the main window of the application (before terminating).
Before destroying a window, an application should save or remove any data associated with the window, and it should release any system resources allocated for the window. If the application does not release the resources, the system will free any resources not freed by the application.
Destroying a window does not affect the window class from which the window is created. New windows can still be created using that class, and any existing windows of that class continue to operate. Destroying a window also destroys the window's descendant windows. The DestroyWindow function sends a WM_DESTROY message first to the window, then to its child windows and descendant windows. In this way, all descendant windows of the window being destroyed are also destroyed.
A window with a window menu receives a WM_CLOSE message when the user clicks Close. By processing this message, an application can prompt the user for confirmation before destroying the window. If the user confirms that the window should be destroyed, the application can call the DestroyWindow function to destroy the window.
If the window being destroyed is the active window, both the active and focus states are transferred to another window. The window that becomes the active window is the next window, as determined by the alt+esc key combination. The new active window then determines which window receives the keyboard focus.