C#基础——virtual

     virtual关键字用于修饰方法、属性、索引器或事件声明，并使它们可以在派生类被重写。

     调用虚方法事时，将为重写成员检查该对象的运行时类型。

     virtual修饰符不能与static、abstract、private或override修饰符一起使用。

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. }