static void Main(String[] args)
{
// 协变
IIndex <Rectangle> rectangles = GenericsRectangle.GetRectangle();
IIndex <Shape> shapes = rectangles;
// 使用接口中的索引器和Count属性
for (int i = 0; i < shapes.count; i++)
{
Console.WriteLine(shapes[i]);
}
// 抗变
IDisplay <Shape> shapeDisplay = new ShapeDisplay();
IDisplay <Rectangle> rectangleDisplay = shapeDisplay;
rectangleDisplay.Show(rectangles[0]);
Console.Read();
}
public static GenericsRectangle GetRectangle()
{
// ??为合并运算符：若gr为null将调用运算符右侧以创
// 建一个GenericsRectangle实例，并将其赋给变量gr
return(gr ?? (gr = new GenericsRectangle()));
}
