How to handle a class you want to extend which is sealed in the .NET library?

Question

I was reading somewhere about how to handle the issue of wanting to extend a sealed class in the .NET Framework library.

This is often a common and useful task to do

Answer 1

No, you can't extend a sealed class in any legitimate way.

TypeMock allows you to mock sealed classes, but I doubt that they'd encourage you to use the same technique for production code.

If a type has been sealed, that means the class designer has not designed it for inheritance. Using it for inheritance at that point may well cause you lots of pain, either now or when the implementation is changed at a later date.

Prefer composition to inheritance - it's a lot more robust, in my experience. See item 16 in "Effective Java (2nd edition)" for more on this.