为什么在C＃中将静态类声明为密封和抽象？

Question

This MSDN article states that static classes should be declared as sealed and abstract. I was under the impression that static classes were already sealed. Why would you also need to declare a static class as sealed?

Answer 1

I think the pertinent bit from that article is:

"Do declare static classes as sealed and abstract, and add a private instance constructor, if your programming language does not have built-in support for static classes. "

Remember, the .NET framework supports a lot of different languages.

Answer 2

C# v1 did not allow 'static' keyword on class. So if you have a class with only static methods, it was suggested to declare it 'sealed', 'abstract' and make constructor private. This means that no one can instantiate your class or try to inherit from it. [It doesn't make sense to inherit from a class which has only static methods.]

C# v2 allowed the static keyword on a class so you don't have to use the above trick to enforce correct usage.

Answer 3

• One of the effects of marking a class as abstract is that it cannot be instantiated.
• One of the effects of marking a class as sealed is that is cannot be inherited.

That's what a static class actually is -- a class that cannot be instantiated and that cannot be inherited.

So the article doesn't state you should mark your static classes as abstract and sealed additionally, but this is the way how static classes are represented in IL.

(Other languages may allow you to do this if they do not have a static keyword, although I doubt it, because an abstract sealed class doesn't make a lot of sense semantically.)

Answer 4

似乎是说如果语言不支持静态类，则使用私有构造函数将类声明为密封和抽象是静态类的替代。