Why aren't classes sealed by default?

Question

I was just wondering, since the sealed keyword\'s existence indicates that it\'s the class author\'s decision as to whether other classes are allowed to inh

Answer 1

Merely deriving from an unsealed class doesn't change the class's behavior. The worst that can happen is that a new version of the base class will add a member with the same name as the deriving class (in which case there will just be a compiler warning saying you should use the new or override modifier) or the base class is sealed (which is a design no-no if the class has already been released into the wild). Arbitrary sublassing still complies with the Liskov Substitution Principle.

The reason that members are not overridable by default in C# is that because overriding a method can change the base class's behaviour in a way that the base class's author didn't anticipate. By making it explicitly abstract or virtual, it's saying that the author is aware that that it can change or is otherwise beyond their control and the author should have taken this into account.