How do you implement the Decorator pattern if you can't inherit from the class you want to decorate?

Question

This is more a design question... You can't do a decorate design pattern if:

1) The object is marked "sealed" meaning you can't extend from it. 2) or you want to override a method but its not virtual.

What can you do then? Taken that you can't change the class source code if you don't have the source code (like a 3rd party library).

Answer 1

Inheritance isn't necessary for a Decorator pattern. The simplest case is if the object you want to decorate implements an interface. Then your decorator can simply implement the same interface:

[   IWindow   ]
[ + Draw()    ]
---------------
  |
  +--- [   Window    ]
  |    [ + Draw()    ]
  |
  |
  +--- [ DecoratedWindow ]
       [ + Draw()        ]
       -------------------
         |
         +--- [ BorderDecorator ]
         |
         +--- [ VerticalScrollbarDecorator ]
         |
         +--- [ HorizontalScrollbarDecorator ]

Now you can just do:

IWindow w = new BorderDecorator(
             new HorizontalScrollBarDecorator(
              new VerticalScrollBarDecorator(
               new Window(80, 24))));

// This is a window with a border and scrollbars, even though
// the original Window class has no idea what those are.
w.Draw();

The specifics of what are possible will depend on the exact nature of the class you're trying to decorate.