用属性扩展密封类

Question

I want to extend the Rectangle class. Currently this class has the properties left , right , ... and I want to add the properties topLeft , topRight , ...

I know I could create some extension methods like

public static Point TopLeft(this Rectangle rect)
{
    return new Point(rect.Left, rect.Top);
}

but I would like to add this as a property. I thought about inheriting from Rectangle and adding the missing information

internal class Rect : Rectangle
{
    public Point TopLeft
    {
        get
        {
            return new Point(X, Y);
        }
    }

    public Point TopRight
    {
        get
        {
            return new Point(X + Width, Y);
        }
    }
}

but Rectangle is a sealed class.

cannot derive from sealed type 'Rectangle'

So it is not possible to extend this class?

Answer 1

You can use the adapter pattern :

internal class RectAdapter
{  
    private Rect _rect;

    public RectAdapter(Rectangle rect)
    {
        _rect = rect;
    }

    public Point TopLeft
    {
        get
        {
            return new Point(_rect.X, _rect.Y);
        }
    }

    public Point TopRight
    {
        get
        {
            return new Point(_rect.X + _rect.Width, _rect.Y);
        }
    }
}

You can't inherit from the Rectangle but you can take it as a constructor parameter. And if you don't want to override other behaviour just delegate them to Rectangle by using _rect , for example:

public void Intersect(Rectangle rect) => _rect.Intersect(rect);