c# - 为什么我不能写 Nullable>？

Question

Nullable<T> 的定义是:

[SerializableAttribute]
public struct Nullable<T> where T : struct, new()

约束where T : struct暗示 T只能是值类型。所以我很清楚我不能写:

Nullable<string> a; //error. makes sense to me

因为 string是引用类型，不是值类型。但是我真的不明白为什么我不能写

Nullable<Nullable<int>> b; //error. but why?

为什么不允许？毕竟，Nullable<int>是一个值类型，因此，它可以是 Nullablle<T> 的类型参数.

当我在 ideone 上编译它时，它给出了这个错误(ideone):

error CS0453: The type 'int?' must be a non-nullable value type in order to use it as type parameter 'T' in the generic type or method 'System.Nullable' Compilation failed: 1 error(s), 0 warnings

Answer 1

因为它在 C# 规范中(第 4.4.4 节):

If the constraint is the value type constraint (struct), the type A must satisfy one of the following:

• A is a struct type or enum type, but not a nullable type. Note that System.ValueType and System.Enum are reference types that do not satisfy this constraint.
• A is a type parameter having the value type constraint (§10.1.5).