Subclassing enables an application to intercept and process messages sent or posted to a window before that window has a chance to process them. Subclassing most often is used to add functionality to a particular window or to alter a window's default behavior.

An application subclasses a window by using the WinSubclassWindow function to replace the window's original window procedure with an application-defined window procedure. Thereafter, the new window procedure processes any messages that are sent or posted to the window. If the new window procedure does not process a particular message, it must pass the message to the original window procedure, not to WinDefWindowProc, for default processing.