CSharp - Use virtual and override to achieve Polymorphism

Introduction

In the following code we tag the parent class (Vehicle) method with virtual and Child class (Bus) method with override.

Demo

using System;

class Vehicle/* www.j  a  v a2s .c  om*/
{
    public virtual void ShowMe()
    {
        Console.WriteLine("Inside Vehicle.ShowMe");
    }
}
class Bus : Vehicle
{
    public override void ShowMe()
    {
        Console.WriteLine("Inside Bus.ShowMe");
    }
    public void BusSpecificMethod()
    {
        Console.WriteLine("Inside Bus.ShowMe");
    }
}
class Program
{
    static void Main(string[] args)
    {
        Vehicle obVehicle = new Bus();
        obVehicle.ShowMe();//Inside Bus.ShowMe
                           // obVehicle.BusSpecificMethod();//Error
                           //Bus obBus = new Vehicle();//Error
    }
}

Result

Analysis

The child class method is invoked, not the parent class method, since we tagged ShowMe() method in the Vehicle class as virtual.

When we call a child's method via a base class reference, the compiler uses the base type reference to invoke the child's method.

By marking a method in the base class as virtual, we intend to achieve polymorphism.

In this way we intentionally redefine/override the method in the child classes.

In the child class, by tagging a method with the keyword override, we tells the compiler that we will redefine the corresponding virtual method.

Note

In C#, all methods are by default non-virtual.

But, in Java, they are virtual by default.

In C# we need to tag the keyword override to avoid any unconscious overriding.