C # basic -- virtual

Virtual keywords are used to modify methods, attributes, indexers, or event declarations, and enable them to be overwritten in a derived class.

When a virtual method is called, The runtime type of the object is checked for the override member.

Virtual modifiers cannot be used with static, abstract, private, or override modifiers.

1. class TestClass{    public class Dimensions    {        public const double PI = Math.PI;        protected double x, y;        public Dimensions()        {        }        public Dimensions(double x, double y)        {            this.x = x;            this.y = y;        }        public virtual double Area()        {            return x * y;        }    }

2.     public class Circle : Dimensions    {        public Circle(double r) : base(r, 0)        {        }        public override double Area()        {            return PI * x * x;        }    }    class Sphere : Dimensions    {        public Sphere(double r) : base(r, 0)        {        }        public override double Area()        {            return 4 * PI * x * x;        }    }    class Cylinder : Dimensions    {        public Cylinder(double r, double h) : base(r, h)        {        }        public override double Area()        {            return 2 * PI * x * x + 2 * PI * x * y;        }    }    static void Main()    {        double r = 3.0, h = 5.0;        Dimensions c = new Circle(r);        Dimensions s = new Sphere(r);        Dimensions l = new Cylinder(r, h);        Console.WriteLine("Area of Circle   = {0:F2}", c.Area());        Console.WriteLine("Area of Sphere   = {0:F2}", s.Area());        Console.WriteLine("Area of Cylinder = {0:F2}", l.Area());        Console.ReadLine();    }

3. }