Besides standard forms, Visual Basic also supports Multiple Document Interface (MDI) forms. An MDI form appears in Figure 4.2.
You can see that an MDI form closely resembles a standard form, with one major difference—the client area of an MDI form acts as a kind of corral for other forms. That is, an MDI form, also called an MDI parent form, can display MDI children in it, which is how the multiple document interface works. In Figure 4.2, we have five documents open in the MDI form. (We'll create this application later in this chapter.)
That's the third type of form you can have in Visual Basic—MDI child forms. These forms appear in MDI child windows, but otherwise are very similar to standard forms. In fact, MDI forms and MDI child forms are both based on the standard System.Windows.Forms namespace like other Windows forms—you make forms into MDI parents and children by setting the IsMdiContainer and MdiParent properties, as we'll see in this chapter.
Those, then, are the three types of Windows forms available to us in Visual Basic: standard forms, MDI forms, and MDI child forms. We'll work with all of them here.