C#协变抗变

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Example
{
    public class Shape         //形状基类
    {
        public double Width { get; set; }
        public double Height { get; set; }
        
    }
    public class Rectangle : Shape   //矩形子类
    {
        public void Sum()
        {
            Console.WriteLine(Width + Height);
        }
    }
    public interface IIndex<out T>      //接口，通过不同的类实现接口的内容，通过不同类的接口实现部分来达到接口的多态功能
    {   //接口是协变的，即同子类转换为父类的方向是一致的，意味着返回类型只能是T
        T this[int index] { get; }       //this代表接口
        int Count { get; }
    }
    public interface IDisplay<in T>    //泛型接口是抗变的，T只能做为其方法的输入
    {
        void Show(T item);
    }
    public class RectangleCollection : IIndex<Rectangle>
    {
        private Rectangle[] data = new Rectangle[3]
        {
            new Rectangle{Height=2,Width=5},
            new Rectangle{Height=3,Width=7},
            new Rectangle{Height=4.5,Width=2.9}
        };
        public static RectangleCollection GetRectangles()
        {
            return new RectangleCollection();
        }
        public Rectangle this[int index]
        {
            get
            {
                if (index < 0 || index > data.Length)
                {
                    throw new ArgumentOutOfRangeException("index");
                }
                return data[index];
            }
        }
        public int Count
        {
            get
            {
                return data.Length;
            }
        }
    }
    public class ShapeDisplay : IDisplay<Shape>
    {
        public void Show(Shape s)
        {
            Console.WriteLine("{0}Width:{1},Height:{2}", s.GetType().Name, s.Width, s.Height);
        }
    }
    class Program
    {
        static void Main()
        {
            RectangleCollection rectangles = RectangleCollection.GetRectangles();  //建立一个类实例
            IIndex<Shape> shapes = rectangles;     //实现接口，此时接口的内容相当于下面注释掉的代码
            for (int i = 0; i < shapes.Count; i++)
            {
                //shapes[i]为Rectangle类型
                Console.WriteLine("{0},{1},{2}", shapes[i].Height, shapes[i].Width, shapes[i].GetType());
            }
        /*public Shape this[int index]
        {
            get
            {
                if (index < 0 || index > data.Length)
                {
                    throw new ArgumentOutOfRangeException("index");
                }
                return data[index];
            }
        }
        public int Count
        {
            get
            {
                return data.Length;
            }
        }*/
            IDisplay<Shape> shapeDisplay = new ShapeDisplay();   //建立一个实例,此时接口函数为void Show(Shape item);
            IDisplay<Rectangle> rectangleDisplay = shapeDisplay;//此时接口函数转换为void Show(Rectangle item);
            rectangleDisplay.Show(rectangles[0]);
        }
    }
}