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C# Polymorphism


Polymorphism and Overriding Methods

Polymorphism means "many forms", and it occurs when we have many classes that are related to each other by inheritance.

Like we specified in the previous chapter; Inheritance lets us inherit fields and methods from another class. Polymorphism uses those methods to perform different tasks. This allows us to perform a single action in different ways.

For example, think of a base class called Animal that has a method called animalSound(). Derived classes of Animals could be Pigs, Cats, Dogs, Birds - And they also have their own implementation of an animal sound (the pig oinks, and the cat meows, etc.):

Example

class Animal  // Base class (parent) 
{
  public void animalSound() 
  {
    Console.WriteLine("The animal makes a sound");
  }
}

class Pig : Animal  // Derived class (child) 
{
  public void animalSound() 
  {
    Console.WriteLine("The pig says: wee wee");
  }
}

class Dog : Animal  // Derived class (child) 
{
  public void animalSound() 
  {
    Console.WriteLine("The dog says: bow wow");
  }
}

Remember from the Inheritance chapter that we use the : symbol to inherit from a class.

Now we can create Pig and Dog objects and call the animalSound() method on both of them:

Example

class Animal  // Base class (parent) 
{
  public void animalSound() 
  {
    Console.WriteLine("The animal makes a sound");
  }
}

class Pig : Animal  // Derived class (child) 
{
  public void animalSound() 
  {
    Console.WriteLine("The pig says: wee wee");
  }
}

class Dog : Animal  // Derived class (child) 
{
  public void animalSound() 
  {
    Console.WriteLine("The dog says: bow wow");
  }
}

class Program 
{
  static void Main(string[] args) 
  {
    Animal myAnimal = new Animal();  // Create a Animal object
    Animal myPig = new Pig();  // Create a Pig object
    Animal myDog = new Dog();  // Create a Dog object

    myAnimal.animalSound();
    myPig.animalSound();
    myDog.animalSound();
  }
}

The output will be:

The animal makes a sound
The animal makes a sound
The animal makes a sound

Try it Yourself »

Not The Output I Was Looking For

The output from the example above was probably not what you expected. That is because the base class method overrides the derived class method, when they share the same name.

However, C# provides an option to override the base class method, by adding the virtual keyword to the method inside the base class, and by using the override keyword for each derived class methods:

Example

class Animal  // Base class (parent) 
{
  public virtual void animalSound() 
  {
    Console.WriteLine("The animal makes a sound");
  }
}

class Pig : Animal  // Derived class (child) 
{
  public override void animalSound() 
  {
    Console.WriteLine("The pig says: wee wee");
  }
}

class Dog : Animal  // Derived class (child) 
{
  public override void animalSound() 
  {
    Console.WriteLine("The dog says: bow wow");
  }
}

class Program 
{
  static void Main(string[] args) 
  {
    Animal myAnimal = new Animal();  // Create a Animal object
    Animal myPig = new Pig();  // Create a Pig object
    Animal myDog = new Dog();  // Create a Dog object

    myAnimal.animalSound();
    myPig.animalSound();
    myDog.animalSound();
  }
}

The output will be:

The animal makes a sound
The pig says: wee wee
The dog says: bow wow

Try it Yourself »

Why And When To Use "Inheritance" and "Polymorphism"?

- It is useful for code reusability: reuse fields and methods of an existing class when you create a new class.