5.7 Nesting Classes

Classes have members, and it is entirely possible for the member 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 int.

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. 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 Outer is the outer class, and Nested is the (public) inner class, refer to Nested as Outer.Nested, with the outer class acting (more or less) as a namespace or scope.

Java programmers take note: Nested classes are roughly equivalent to static inner classes; there is no C# equivalent to Java's nonstatic inner classes.

Example 5-6 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 5-6. Using a nested class

using System;
using System.Text;

public class Fraction
{
    private int numerator;
    private int denominator;

    public Fraction(int numerator, int denominator)
    {
        this.numerator=numerator;
        this.denominator=denominator;
    }

    public override string ToString( )
    {
        return String.Format("{0}/{1}", 
            numerator, denominator);
    }

    
internal class FractionArtist
    {
        public void Draw(Fraction f)
        {
            Console.WriteLine("Drawing the numerator: {0}",
               f.numerator);
            Console.WriteLine("Drawing the denominator: {0}", 
               f.denominator);
        }
    }

}


public class Tester
{
  static void Main( )
  {
        Fraction f1 = new Fraction(3,4);
        Console.WriteLine("f1: {0}", f1.ToString( ));

        Fraction.FractionArtist fa = new Fraction.FractionArtist( );
        fa.Draw(f1);
  }
}

The nested class is shown in bold. The FractionArtist class provides only a single member, the Draw( ) method. What is particularly interesting is that Draw( ) has access to the private data members f.numerator and f.denominator, to which it would not have had access if it were not a nested class.

Notice in Main( ) that to declare an instance of this nested class, you must specify the type name of the outer class:

Fraction.FractionArtist fa = new Fraction.FractionArtist( );

FractionArtist is scoped to within the Fraction class.