Polymorphism in C# with example

Polymorphism is derived from two Latin words, 1. Poly means many 2. Marphose means forms. By using inheritance, a class can be used as many types, say we have a Person which may be a Professor, may be a Student, may be Clerk. So in this article we will see some example of Polymorphism.

When we derive a class from base class it inherits all the method, properties, fields and events of the base class. To change the behavior or data of base class we have two ways, either we can hide the base method by using new key word or override the base method by using override key word.

Let's create a class Person which we will use as base class for other classes, note we used virtual for Display method so we can override it in our derived classes.

public class Person
{
    public virtual void Display()
    {
        Console.WriteLine("I am a person!");
    }
}

Now create some other classes derived from Person class and override display method in every class.

public class Professor: Person
{
    public override void Display()
    {
        Console.WriteLine("I am a professor!");
    }
}

public class Student : Person
{
    public override void Display()
    {
        Console.WriteLine("I am a student!");
    }
}

public class Clerk : Person
{
    public override void Display()
    {
        Console.WriteLine("I am a clerk!");
    }
}

Now we will create an object of person and initialize with different classes one by one and call their display method

Person person = new Person();
person.Display();

person = new Professor();
person.Display();    

person = new Student();
person.Display();

person = new Clerk();
person.Display();

What would be the out put, any guess, do you think it will display 4 times "I am a person!", now, here is the out put

I am a person!
I am a professor!
I am a student!
I am a clerk!

So in this example we see person is behaving like a person, professor, student and then as a clerk. This is the power of polymorphism, a single object behaves in different forms.

As we know, to hide a parent method we use new key word so let's change our student class and don't override but use new key word see this

public class Student : Person
{
    public new void Display()
    {
        Console.WriteLine("I am a student!");
    }
}

Our other classes are unchanged, so let's check the output of following

Person person = new Person();
person.Display();

person = new Professor();
person.Display();    

person = new Student();
person.Display();

person = new Clerk();
person.Display();
// Output
I am a person!
I am a professor!
I am a person!*  
I am a clerk!

* Person don't know anything about the display method of student because it hides it. only when we create an object of student and initialize with student then it's display method will show I am a student.

For overloading and overriding see my other article Overloading and Overriding in C# with example

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