C# 虚方法与抽象类

1.虚方法
将父类的方法标记为虚方法 ，使用关键字 virtual，这个函数可以被子类重新写一个遍。

namespace _10多态练习
{
    class Program
    {
        static void Main(string[] args)
        {
            RealDuck rd = new RealDuck();
            MuDuck md = new MuDuck();
            XPDuck xd = new XPDuck();
            RealDuck[] ducks = { rd, md, xd };
            for (int i = 0; i < ducks.Length; i++)
            {
                ducks[i].Bark();
            }
            Console.ReadKey();

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

    public class MuDuck : RealDuck
    {
        public override void Bark()
        {
            Console.WriteLine("木头鸭子吱吱叫");
        }
    }

    public class XPDuck : RealDuck
    {
        public override void Bark()
        {
            Console.WriteLine("橡皮鸭子唧唧叫");
        }
    }
}

2.抽象类
当父类中的方法不知道如何去实现的时候，可以考虑将父类写成抽象类，将方法写成抽象方法。

namespace _12_抽象类
{
    class Program
    {
        static void Main(string[] args)
        {
            //使用多态求矩形的面积和周长以及圆形的面积和周长
            Shape shape = new Square(5, 6); //new Circle(5);
            double area = shape.GetArea();
            double perimeter = shape.GetPerimeter();
            Console.WriteLine("这个形状的面积是{0},周长是{1}", area, perimeter);
            Console.ReadKey();

        }
    }

    public abstract class Shape
    {
        public abstract double GetArea();
        public abstract double GetPerimeter();
    }
    public class Circle : Shape
    {

        private double _r;
        public double R
        {
            get { return _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;
        }
    }
    public class Square : Shape
    {
        private double _height;

        public double Height
        {
            get { return _height; }
            set { _height = value; }
        }

        private double _width;

        public double Width
        {
            get { return _width; }
            set { _width = value; }
        }

        public Square(double height, double width)
        {
            this.Height = height;
            this.Width = width;
        }

        public override double GetArea()
        {
            return this.Height * this.Width;
        }

        public override double GetPerimeter()
        {
            return (this.Height + this.Width) * 2;
        }
    }

}