public class Animal

     {

         public string Name { get; set; }

         public virtual void Eat()

         {

             Console.WriteLine("{0}正在吃草",Name);

         }

     }

     public class Sheep : Animal

     {

         public Sheep(){ Name = "羊"; }

         public override void Eat()

         {

             base.Eat();

             Console.WriteLine("吃草");

         }

     }

     public class Tigger : Animal

     {

         public Tigger() { Name = "老虎"; }

         public override void Eat()

         {

             base.Eat();

             Console.WriteLine("老虎吃羊");

         }

     }

   Animal animal1 = new Sheep();

            Animal animal2 = new Tigger();

            animal1.Eat();

            animal2.Eat();

public abstract class Shape

    {

        public const double Pi = 3.14;

        protected double x, y;

        public Shape()

        {

            x = y = 0;

        }

        public Shape(double x,double y)

        {

            this.x = x;

            this.y = y;

        }

        public abstract double Area(); 

    }

    //矩形

    public class Rectangle : Shape

    {

        public Rectangle() : base() { }//使用基类的构造函数

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

        public override double Area()

        {

            return x * y;

        }

    }

    //椭圆

    public class Ellipase : Shape

    {

        public Ellipase(double x, double y) : base(x, y) { }

        public override double Area()

        {

            return x * y * Pi;

        }

    }

    //圆形

    public class Round : Shape

    {

        public Round(double x) : base(x, 0) { }

        public override double Area()

        {

            return x*x*Pi;

        }

    }

 double x = 2;

            double y = 3;

            Shape shape1 = new Rectangle(x,y);

            Console.WriteLine("Rectangle:"+ shape1.Area());

            Shape shape2= new Round(x);

            Console.WriteLine("Round:"+ shape2.Area());

            Shape shape3 = new Ellipase(x,y);

            Console.WriteLine("Ellipase:"+shape3.Area());