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扩展。

1、协变：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[3]
        {
            new Rectangle{Width=2,Height=3},
            new Rectangle{Width=3,Height=7},
            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 < 0 || 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("{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[0]);
            Console.ReadKey();
        }

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