C# Polymorphism

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.):

Method Overriding

To override a base class method, you must add the virtual keyword to the method in the base class, and use the override keyword for each derived class method:

Examples

Polymorphism Example

Different implementations of the same method.

using System;

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();
  }
}

No Override

Difference without virtual/override.

using System;

class Animal 
{
    public void sleep() { Console.WriteLine("Sleeping"); }
}

class Dog : Animal
{
    // Hides the base method without override (warning)
    public new void sleep() { Console.WriteLine("Dog Sleeping"); }
}

class Program
{
    static void Main()
    {
        Animal a = new Dog();
        a.sleep(); // Calls Animal's sleep because not virtual/override
        
        Dog d = new Dog();
        d.sleep(); // Calls Dog's sleep
    }
}