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("width:{0},height:{1}", Width, Height);
}
}
public class Rectangle : Shape
{
}
协变:
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} width:{1},height:{2}", item.GetType().Name, item.Width, item.Height);
}
}
抗变: