Runtime Polymorphism: Method Overriding

Run-time polymorphism is also known as inheritance-based polymorphism or method overriding.

Inheritance allows you to inherit a base class into a derived class and all the public members of the base class automatically become members of the derived class. However, you can redefine the base class's member in the derived class to provide a different implementation than the base class. This is called method overriding that also known as runtime polymorphism.

In C#, by default, all the members of a class are sealed and cannot be redefined in the derived class. Use the virtual keyword with a member of the base class to make it overridable, and use the override keyword in the derived class to indicate that this member of the base class is being redefined in the derived class

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

class Employee : Person
{
    public override void Greet()
    {
        Console.WriteLine("Hello! I am an employee.");
    }
}

As you can see, Greet() method in the Person class is defined with the virtual keyword. It means this method can be overridden in the derived class using the override keyword. In the Employee class, the Greet() method is redefined with the override keyword. Thus, the derived class extends the base class's method.

Now, the question is which method will be called, the base class's method or the derived class method? Well, it depends on the type of object. Can you guess the output of the following program?

Person p1 = new Person();
p1.Greet();

Person p2 = new Employee();
p2.Greet();

Employee emp = new Employee();
emp.Greet();

I am a human!
I am a Manager!
I am a Manager!

C# will call the method depending on the type of the object not the type of the variable. If you create an object of the Person class then it will call the Greet() method of the Person class and if you create an object of the Employee class then it will call the Greet() method of the Employee class no matter to which type of variable it assigned to.

As you learned in the previous chapter the C# compiler decides which methods to call at the compile time in the compile-time polymorphism. In the run time polymorphism, it will be decided at run time depending upon the type of an object.

To understand why method overriding is called the runtime polymorphism, look at the following example.

class Program
{
    public static void Display(Person p){ 
        p.Greet();
    }

    public static void Main()
    {
        Person p1 = new Person();
        Display(p1);
            
        Person p2 = new Employee();
        Display(p2);
            
        Employee emp = new Employee();
        Display(emp);
    }
}

I am a human!
I am the Manager!
I am the Manager!

In the above example, the Display() method takes a parameter of the Person type. Thus, you can pass an object of the Person type or the Employee type when you call the Display() method. The Display() method does not know the type of parameter you passed at compile time. It can be anything at runtime. That's why method overriding is called run-time polymorphism.

Rules for Overriding:

• A method, property, indexer, or event can be overridden in the derived class.
• Static methods cannot be overridden.
• Must use virtual keyword in the base class methods to indicate that the methods can be overridden.
• Must use the override keyword in the derived class to override the base class method.

Learn more about virual and override keywords.

What will happen if we don't use virtual and override keywords and redefine the base class method in the derived class? Learn about it in the next chapter.