Programming Visual Basic .NET, Second Edition

Nested Classes

Classes have members, and it is entirely possible for the members of a class to be another user-defined type. Thus, a Button class might have a member of type Location, and a Location class might contain members of type Point. Finally, Point might contain members of type Integer.

At times, the contained class might exist only to serve the outer class, and there might be no reason for it to be otherwise visible. (In short, the contained class acts as a helper class.) You can define the helper class within the definition of the outer class. The contained, inner class is called a nested class, and the class that contains it is called, simply, the outer class.

Nested classes have the advantage of access to all the members of the outer class. That is, a method of a nested class can access private members of the outer class.

In addition, the nested class can be hidden from all other classes—that is, it can be private to the outer class.

Finally, a nested class that is public is accessed within the scope of the outer class. If Button is the outer class, and Location is the (public) inner class, you refer to Location as Button.Location, with the outer class (Button) acting (more or less) as a namespace or scope.

Example 6-6 features a nested class of Fraction named FractionArtist. The job of FractionArtist is to render the fraction on the console. In this example, the rendering is handled by a pair of simple WriteLine( ) statements.

Example 6-6. Using a nested class

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