What does class? (class with question mark) mean in a C# generic type constraint?

问题

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?
{
}

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

回答1:

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

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

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

 Add<T>(T tmp);

You document, it's OK to Add null;

回答2:

To allow your class return null value without CS864 warning you can rewrite your code a little bit

public T? Foo<T>(T obj)
    where T : class
{
    return null;
}

The question about class? is open GitHub issue right now, have a look here. class? constraint isn't implemented yet and therefore compiler threats it like regular class and generate A null literal introduces a null value when 'T' is a non-nullable reference type for now.

You can also refer to the The issue wit T? section in this article to see some details. The code sample above is recommended way to deal with nullable generic type

来源：https://stackoverflow.com/questions/59630532/what-does-class-class-with-question-mark-mean-in-a-c-sharp-generic-type-const