MDI Changes in Visual Basic .NET
In Visual Basic 6.0, multiple-document interface (MDI) applications were created by adding an MDI form to a project and setting the MDIChild property of any child forms. In Visual Basic .NET, any form can be made an MDI parent by setting the IsMdiContainer property to true.
Properties and methods that applied to Visual Basic 6.0 MDI forms have changed in Visual Basic .NET. For more information, see MDIForm Object Changes in Visual Basic .NET.
The behavior of MDI applications has also changed. In Visual Basic 6.0, an MDI application that also contained a form that was not an MDI child would not end until that form was closed, even if the MDI parent was closed. In Visual Basic .NET, the application will end when the startup form is closed, regardless of any non-MDI forms in the application.