Programming Visual Basic .NET

MDI Applications

Multiple document interface (MDI) applications permit more than one document to be open at a time. This is in contrast to single document interface (SDI) applications, which can manipulate only one document at a time. Visual Studio .NET is an example of an MDI application—many source files and design views can be open at once. In contrast, Notepad is an example of an SDI application—opening a document closes any previously opened document.

There is more to MDI applications than their ability to have multiple files open at once. The Microsoft Windows platform SDK specifies several UI behaviors that MDI applications should implement. The Windows operating system provides support for these behaviors, and this support is exposed through Windows Forms as well.

Parent and Child Forms

MDI applications consist of a main form, which does not itself display any data, and one or more child forms, which appear only within the main form and are used for displaying documents. The main form is called the MDI parent, and the child forms are called the MDI children.

The Form class has two properties that control whether a given form is an MDI parent, MDI child, or neither. The Boolean IsMdiContainer property determines whether a form behaves as an MDI parent. The MdiParent property (which is of type Form) controls whether a form behaves as an MDI child. Setting the MdiParent property of a form to reference the application’s MDI parent form makes the form an MDI child form. ...