VB.NET 2005 Tutorials: Creating Multiple-Document Interface (MDI) Applications In this tutorial you will learn about Creating Multiple Document Interface (MDI) Applications. In most real time applications we often find that multiple windows open within another window. This kind of need is satisfied by implementing Multiple Document Interface (MDI) Applications. You can create a class and designate it as a MDI window by setting its IsMDIContainer Property as true. Set the value for WindowState as maximized.