C#泛型中的抗变和协变

在.net4之前，泛型接口是不变的。.net4通过协变和抗变为泛型接口和泛型委托添加了一个重要的拓展

1、抗变：如果泛型类型用out关键字标注，泛型接口就是协变的。这也意味着返回类型只能是T。

实例：

 static void Main(string[] args)
         {
             IIndex<Rectangle> rectangles = RectangleCollection.GetRectangles();
             IIndex<Shape> shapes = rectangles;
             Console.ReadKey();
         }

 public interface IIndex<out T>
     {
         T this[int index] { get; }
         int Count { get; }
     }
     public class RectangleCollection : IIndex<Rectangle>
     {
         private Rectangle[] data = new Rectangle[]
         {
             new Rectangle{Width=,Height=},
             new Rectangle{Width=,Height=},
             new Rectangle{Width=4.5,Height=2.9}
         };
         private static RectangleCollection coll;

         public static RectangleCollection GetRectangles()
         {
             return coll ?? (coll = new RectangleCollection());
         }
         public Rectangle this[int index]
         {
             get
             {
                 if (index <  || index > data.Length)
                 {
                     throw new ArgumentOutOfRangeException("index");
                 }
                 return data[index];
             }
         }
         public int Count
         {
             get
             {
                 return data.Length;
             }
         }
     }
     public class Shape
     {
         public double Width { get; set; }
         public double Height { get; set; }
         public override string ToString()
         {
             return String.Format("width:{0},height:{1}", Width, Height);
         }
     }
     public class Rectangle : Shape
     {

     }

2、抗变：如果泛型类型用in关键字，泛型接口就是抗变得。这样，接口的只能把泛型类型T用作方法的输入

实例：

static void Main(string[] args)
        {
            IIndex<Rectangle> rectangles = RectangleCollection.GetRectangles();
            IDisplay<Shape> shapeDisplay = new ShapeDisplay();
            IDisplay<Rectangle> rectangleDisplay = shapeDisplay;
            rectangleDisplay.Show(rectangles[]);
            Console.ReadKey();
        }

public interface IDisplay<in T>
    {
        void Show(T item);
    }
    public class ShapeDisplay : IDisplay<Shape>
    {
        public void Show(Shape item)
        {
            Console.WriteLine("{0} width:{1},height:{2}", item.GetType().Name, item.Width, item.Height);
        }
    }