Cannot inherit Shapes

问题:

Why I can't use a class that inherits a Shapes class?

I need to extend the Rectangle class with some methods, but i want to use this class in the same way I use a Shape, how can I do?

回答1:

As Jon pointed out, Rectangle is sealed.

Depending on what you're trying to do, a couple options are:

1. You can extend Shape with your own class that contains Rectangle, and augment the functionality through composition. These objects wouldn't be considered Rectangles from an "is" check.

2. You can write extension methods for Rectangle, and then you can use them on any Rectangle. Then the objects would still be considered Rectangles.

e.g.

public static class RectangleExtensions {
    public static bool IsSquare(this Rectangle r) {
        return r.Width == r.Height;
    }
}

回答2:

You can write a class which derives from Shape. You can't write a class which derives from Rectangle, because that's sealed.