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

I can't recall having heard a rationale for the decision to have classes not sealed by default. However, there are certainly quite a few people who believe that C# should have been spec'ed to have sealed be the default:

http://codebetter.com/blogs/patricksmacchia/archive/2008/01/05/rambling-on-the-sealed-keyword.aspx

Answer 2

In my opinion there should be no default syntax, that way you always write explicitly what you want. This forces the coder to understand/think more.

If you want a class to be inheritable then you write

public extensible class MyClass

otherwise

public sealed class MyClass

BTW I think the same should go with access modifiers, disallow default access modifiers.

Answer 3

I'd say it was just a mistake. I know many people (including myself) who believe that classes should indeed be sealed by default. There are at least a couple of people in the C# design team in that camp. The pendulum has swung somewhat away from inheritance since C# was first designed. (It has its place, of course, but I find myself using it relatively rarely.)

For what it's worth, that's not the only mistake along the lines of being too close to Java: personally I'd rather Equals and GetHashCode weren't in object, and that you needed specific Monitor instances for locking too...

Answer 4

I see two simple reasons:

1. Inheritance is a foundational principle of OO, so disallowing it by default wouldn't be intuitive.
2. The majority of classes are designed to allow inheritance, so allowing inheritance by default saves typing.

Answer 5

sealed classes prevent inheritance and therefore are an OO abombination. see this rant for details ;-)

Answer 6

You could probably make just as many arguments in favor of sealed-by-default as you could against it. If it were the other way around, someone would be posting the opposite question.