多型：

虛方法（virtual    override）

抽象類（abstract  override）

class Program
    {
        static void Main(string[] args)
        {

            //虛方法


            /*RealDuck realDuck = new RealDuck();
            realDuck.Bark();
            MuDuck muDuck = new MuDuck();
            muDuck.Bark();
            XpDuck xpDuck = new XpDuck();
            xpDuck.Bark();
 
*/
            RealDuck[] rd = { new RealDuck(), new MuDuck(), new XpDuck() };
            for(int i = 0; i < rd.Length; i++)
            {
                rd[i].Bark();
            }


            //抽象類練習
            //計算圓形的面積
            XingZhuang xz = new Circle(5);
            double area = xz.GetArea();
            
 
double perimeter = xz.GetPerimeter();
            Console.WriteLine("這個形狀的面積是{0:0.00},周長是{1:0.00}", area, perimeter);

        }
    }
    
    abstract class XingZhuang
    {
        //父類，封裝子類共有的成員
        public abstract double GetArea();
        public abstract double GetPerimeter();
    }


    class Circle : XingZhuang
    {
        
 
private double _r;

        public double R { get => _r; set => _r = value; }

        public Circle(double r)
        {
            this.R = r;
        }

        public override double GetArea()
        {
            return Math.PI*this.R * this.R;
        }

        public override double GetPerimeter()
        {
            return 2*Math.PI * this.R ;
        }
    }



    class RealDuck
    {
    public virtual void Bark()
        {
            Console.WriteLine("嘎嘎叫");
        }
    }

    class MuDuck : RealDuck
    {
        public override void Bark()
        {
            Console.WriteLine("木頭鴨子吱吱叫");
        }
    }
    class XpDuck : RealDuck
    {
        public override void Bark()
        {
            Console.WriteLine("橡皮鴨子唧唧叫");
        }
    }