Why seal a class?

Question

I\'d like to hear what is the motivation behind the bulk of sealed classes in the .Net framework. What is the benefit of sealing a class? I cannot fathom how not allowing in

Answer 1

Classes should either be designed for inheritance or prohibit it. There is a cost to designing for inheritance:

• It can pin down your implementation (you have to declare which methods are going to call which other methods, in case a user overrides one but not the other)
• It reveals your implementation rather than just the effects
• It means you have to think of more possibilities when designing
• Things like Equals are hard to design in an inheritance tree
• It requires more documentation
• An immutable type which is subclassed may become mutable (ick)

Item 17 of Effective Java goes into more details on this - regardless of the fact that it's written in the context of Java, the advice applies to .NET as well.

Personally I wish classes were sealed by default in .NET.