c# - 为什么 null 不是 Nullable<> 类型的实例？

Question

在编写涉及手动类型检查的测试时，我发现 null不评估为 Nullable<> 的实例类型，例如对于类型 Nullable<int> (与 int? 相同)以下陈述是错误的

null is int?

分配null时至int?已验证。为什么会这样？

Answer 1

那么，为什么可空值应该是特殊的？这不代表任何引用类型，句号:

var isObject = null is object;

会给你一个编译时错误:

Error CS0184: The given expression is never of the provided ('object') type

Nullable<T>这是一个转移注意力的事情。

null的类型是 null 类型，并且它可以分配给任何引用类型或可为 null 的类型，因为存在隐式转换(第 2.4.4.6 节)。

这个is根据规范的运算符按以下方式工作(第 7.10.10 节):

The is operator is used to dynamically check if the run-time type of an object is compatible with a given type. The result of the operation E is T, where E is an expression and T is a type, is a boolean value indicating whether E can successfully be converted to type T by a reference conversion, a boxing conversion, or an unboxing conversion. The operation is evaluated as follows, after type arguments have been substituted for all type parameters:

1. If E is an anonymous function, a compile-time error occurs
2. If E is a method group or the null literal, or if the type of E is a reference type or a nullable type and the value of E is null, the result is false. (...)

粗体部分用于澄清。

阅读this和 this answer了解更多信息。