Paris Polyzos' Blog

What confuses sometimes is the use of virtual keyword, so let’s keep some notes in order to remember it.

Definition
The virtual keyword is used to modify a method or property declaration, in which case the method or the property is called a virtual member. The implementation of a virtual member can be changed by an overriding member in a derived class.

Have in mind
You cannot use the virtual modifier with the following modifiers: static, abstract, override.

How to use it
A virtual inherited property can be overridden in a derived class by including a property declaration that uses the override modifier.

When to use it
When you want to support polymorphism. When objects derive from a common class, having some common behavior but there are some small differences at the implementation of each object’s behavior.

Examples

using System;
class TestVirtual
{
   public class Shape
   {
      public const double pi = Math.PI;
      protected double x, y;
      public Shape() { }
      public Shape (double x, double y)
      {
         this.x = x;
         this.y = y;
      }

      //This method can be overriden
      public virtual double Area()
      {
         return x*y;
      }
   }

   public class Circle: Shape
   {
      public Circle(double r): base(r, 0)
      {
      }

     // This method is being overriden
      public override double Area()
      {
         return pi * x * x;
      }
   }

	public class Rectangle: Shape
	{
		public Rectangle(double x, double y): base(x, y){}
	}

   public static void Main()
   {
      double r = 3.0, h = 5.0;
      Shape c = new Circle(r);
      Shape s = new Rectangle(r, h);

      // Display results:
      Console.WriteLine("Area of Circle   = {0:F2}", c.Area());
      Console.WriteLine("Area of Rectangle   = {0:F2}", s.Area());
   }
}