public void OutMethod()
{
//IIndex<Rectangle> rectangles = RectangleCollection.GetRectangles();
//IIndex<Shape> shapes = rectangles; //因为 限定了返回为Rectangle类型 所以赋值给Shape类型没有问题(简写如下)
IIndex <Shape> shapes = RectangleCollection.GetRectangles();
for (int i = 0; i < shapes.Count; ++i)
{
Console.WriteLine(shapes[i]);
}
}
public static RectangleCollection GetRectangles()
{
return(coll ?? (coll = new RectangleCollection()));
}