Why does Nullable not match as a reference type

问题:

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Closed 7 years ago.

Possible Duplicate:
Nullable type as a generic parameter possible?

I came across a very weird thing with generic type constraints. I have a class like this:

public SomeClass<T> where T:class
{
}

However, I've found I can't use nullable types as I'd expect:

new SomeClass<int?>();

I get an error that int? must be a reference type. Is Nullable really just a struct with syntactic sugar to make it look like a reference type?

回答1:

Nullable<T> is a struct (see MSDN) however it is the only struct that does not satisfy the struct constraint. Therefore, you cannot use a Nullable as a generic type parameter when either the class or struct constraints is used.

Nullable<T> is not just a struct with some syntatic sugar. It has special support in the CLR for some of its behavior. For example, it has special boxing behavior. Specifically, a nullable is never boxed. The underlying value is boxed. If the nullable is the null value (HasValue is false) then it is converted to a null reference. Also, conversion operators for any Nullable<T> to Nullable<U> are lifted from the conversions from T to U. These are features you wouldn't be able to implement yourself in .NET 1.0/1.1.