Every class needs a window-procedure address to define the entry point of the window procedure used to process all messages for windows in the class. Windows passes messages to the procedure when it requires the window to carry out tasks, such as painting its client area or responding to input from the user. An application assigns a window procedure to a class by copying its address to the lpfnWndProc member of the WNDCLASSEX structure. For more information, see Window Procedures.