Using the Multiple Document Interface  This section explains how
to perform the following tasks:  To illustrate these tasks, this
section includes examples from Multipad, a typical multiple-
document interface (MDI) application.  Registering Child and
Frame Window Classes  A typical MDI application must register
two window classes: one for its frame window and one for its
child windows. If an application supports more than one type of
document (for example, a spreadsheet and a chart), it must
register a window class for each type.  The class structure for
the frame window is similar to the class structure for the main
window in non–MDI applications. The class structure for the MDI
child windows differs slightly from the structure for child
windows in non–MDI applications as follows:    • The class
structure should have an icon, because the user can minimize an
MDI child window as if it were a normal application window.   •
The menu name should be NULL, because an MDI child window cannot
have its own menu.   •