What is virtual method in C# language and why do you use it?

Answer: A virtual method in C# language is method that have virtual keyword in its declaration. In c# inheritance feature, we may want to allow derived classes to have their own implementation for some of the methods resides into a base class.

For this purpose, we use virtual keyword in method declaration in base class, so, that derive class can override it and implement its own feature.

In below c# code example, we have “Printer” base class that have two method print() and display(). print()method does printing and display() function will have display unit of dimension of 5×5 units.

Here, we want to have a derived class “LaserJet” printer that will use the print() method of base class, but, will have display unit of 10×10.

So, we will declare base class display() method as virtual and override it into derived class and implement new dimension of 10×10.

class Printer
{
    //common print function that all derived classes will use
    //the same feature.
    //Not making it virtual as all derived classes will call this.
    public void print()
    {
        Console.WriteLine("Printer Printing...");
    }

    //Making virtual, so, derived classes can implement thier
    //own feature.
    public virtual void display()
    {
        Console.WriteLine("Printer Display dimention: 5x5\n");
    }


}
class LaserJet : Printer
{

    public override void display()
    {
        Console.WriteLine("LaserJet Display dimention: 10x10");
    }

}


//------------TEST-----------
namespace virtual_method
{
    class Program
    {
        static void Main(string[] args)
        {
            //Base class object
            Printer p = new Printer();
            p.print();//print
            p.display();

            //Derived class behaviour
            Printer lp = new LaserJet();
            lp.print();//use base class print() method

            // will use its(LaserJet) own display() method implementation.
            lp.display();

        }
    }

}

NOTES:

Virtual keyword is different from “abstract” as virtual keyword does not force derived class to implement it, but, abstract forces derived classes to implement it.