什么课? (带问号的类)是指C#泛型类型约束? [英] What does class? (class with question mark) mean in a C# generic type constraint?

问题描述

当我试图找到另一个问题的答案时，我注意到该代码是在C#中编译的:

While I tried to find an answer to another question I noticed that this code compiles in C#:

public void Foo<T>(T obj)
    where T : class?
{
}

我没有设法在文档中找到它的含义.

I did not manage to find in the documentation what it even means.

推荐答案

它强制T必须是可为空的引用类型.

It enforces that T has to be a nullable reference type.

您为T设置的类型必须源自object?.

The type you set in for T, must derive from object?.

这是C#8中的一项新功能，可以将类型明确声明为可为空. 如果有

It's a new feature in C#8, to explictly declare a type as nullable. if you have

 Add<T>(T tmp);

您记录了文档，可以添加null；

You document, it's OK to Add null;

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