c# - When and why would you seal a class?

Question

In C# and C++/CLI the keyword sealed (or NotInheritable in VB) is used to protect a class from any inheritance chance (the class will be non-inheritable). I know that one feature of object-oriented programming is inheritance and I feel that the use of sealed goes against this feature, it stops inheritance. Is there an example that shows the benefit of sealed and when it is important to use it?

Answer 1

1. On a class that implements security features, so that the original object cannot be "impersonated".

2. More generally, I recently exchanged with a person at Microsoft, who told me they tried to limit the inheritance to the places where it really made full sense, because it becomes expensive performance-wise if left untreated.
   The sealed keyword tells the CLR that there is no class further down to look for methods, and that speeds things up.

In most performance-enhancing tools on the market nowadays, you will find a checkbox that will seal all your classes that aren't inherited.
Be careful though, because if you want to allow plugins or assembly discovery through MEF, you will run into problems.