C# 0 The Complete Reference

PART I

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I n h e r i t a n c e  

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PART IPART I

class Derived2 : Base {

  // Override Who() again in another derived class.

  public override void Who() {

    Console.WriteLine("Who() in Derived2");

  }

}

class OverrideDemo {

  static void Main() {

    Base baseOb = new Base();

    Derived1 dOb1 = new Derived1();

    Derived2 dOb2 = new Derived2();

    Base baseRef; // a base class reference

    baseRef = baseOb;

    baseRef.Who();

    baseRef = dOb1;

    baseRef.Who();

    baseRef = dOb2;

    baseRef.Who();

  }

}

The output from the program is shown here:

Who() in Base

Who() in Derived1

Who() in Derived2

This program creates a base class called 

Base

 and two derived classes, called 

Derived1

and

Derived2

.

Base

 declares a method called 

Who( )

, and the derived classes override it. 

Inside the 

Main( )

 method, objects of type 

Base

,

Derived1

, and 

Derived2

 are declared. Also, 

a reference of type 

Base

, called 

baseRef

, is declared. The program then assigns a reference 

to each type of object to 

baseRef

 and uses that reference to call 

Who( )

. As the output 

shows, the version of 

Who( )

 executed is determined by the type of object being referred to 

at the time of the call, not by the class type of 

baseRef

.

It is not necessary to override a virtual method. If a derived class does not provide its 

own version of a virtual method, then the one in the base class is used. For example:

/* When a virtual method is not overridden,

   the base class method is used. */

using System;

class Base {

  // Create virtual method in the base class.

  public virtual void Who() {

    Console.WriteLine("Who() in Base");

  }

}

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class Derived1 : Base {

  // Override Who() in a derived class.

  public override void Who() {

    Console.WriteLine("Who() in Derived1");

  }

}

class Derived2 : Base {

  // This class does not override Who().

}

class NoOverrideDemo {

  static void Main() {

    Base baseOb = new Base();

    Derived1 dOb1 = new Derived1();

    Derived2 dOb2 = new Derived2();

    Base baseRef; // a base class reference

    baseRef = baseOb;

    baseRef.Who();

    baseRef = dOb1;

    baseRef.Who();

    baseRef = dOb2;

    baseRef.Who(); // calls Base's Who()

  }

}

The output from this program is shown here:

Who() in Base

Who() in Derived1

Who() in Base

Here, 

Derived2

 does not override 

Who( )

. Thus, when 

Who( )

 is called on a 

Derived2

object, the 

Who( )

 in 

Base

 is executed.

In the case of a multilevel hierarchy, if a derived class does not override a virtual 

method, then, while moving up the hierarchy, the first override of the method that is 

encountered is the one executed. For example:

/*  In a multilevel hierarchy, the first override of a virtual

    method that is found while moving up the hierarchy is the

    one executed. */

using System;

class Base {

  // Create virtual method in the base class.

  public virtual void Who() {

    Console.WriteLine("Who() in Base");

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