为什么Nullable< T>不匹配通用约束的参考类型 [英] Why does Nullable<T> not match as a reference type for generic constraints

问题描述

可能重复：

Nullable类型作为可能的泛型参数？

我遇到了一个非常奇怪的东西，通用类型约束。我有这样的一个类：

  public SomeClass< T> T：class 
 {
} 
  

然而，我发现我不能像我所期望的那样使用可为null的类型：

  new SomeClass< int？>（）; 
  

我得到一个错误，指出 int？必须是一个参考类型。 Nullable真的只是一个带有语法糖的结构，使它看起来像一个引用类型吗？

解决方案

Nullable< ; T> 是 struct （请参阅 MSDN ），但它是唯一不符合 struct 的 struct $ c>约束。因此，当类或结构体时，不能使用 Nullable 作为泛型类型参数使用约束。

可空<> 合成糖。 CLR对它的一些行为有特别的支持。例如，它有特殊的拳击行为。具体来说，一个可为空的永远不会被装箱。底层价值是盒装的。如果可为空值（HasValue为false），则将其转换为空引用。另外，任何 Nullable 到 Nullable 的转换操作符都从 T 至 U 。这些功能是您无法在.NET 1.0 / 1.1中实现的功能。

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?

解决方案

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.

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