可空类型“int"的默认值是多少?(包括问号)? [英] What is the default value of the nullable type "int?" (including question mark)?

问题描述

在 C# 中，int 类型的类实例变量的默认值是多少??

例如，在下面的代码中，如果 MyNullableInt 从未显式赋值，会有什么值?

For example, in the following code, what value will MyNullableInt have if it is never explicitly assigned?

class MyClass
{
    public int? MyNullableInt;
}

(似乎答案几乎可以肯定是 null 或 0，但它们中的哪一个?)

(It seems likely that the answer is almost certainly either null or 0, but which of those is it?)

推荐答案

int? 的默认值——以及任何使用type?"的可空类型声明 -- 是 null.

The default value for int? -- and for any nullable type that uses the "type?" declaration -- is null.

为什么会这样:

• int? 是 Nullable<T>(其中 T 是 int)，一个结构体.(参考)
• Nullable<T> 类型具有 bool HasValue成员，当 false 时，使 Nullable 实例表现得像"一个 null 值.特别是，Nullable<T>.Equals 方法被覆盖以返回 true 当 HasValue == false 的 Nullable<T> 与实际的 null 值进行比较时.
• 来自 C# 语言规范 11.3.4，结构实例的初始默认值是该结构的所有值类型字段设置为其默认值，并且该结构的所有引用类型字段设置为 null.
• C# 中 bool 变量的默认值为 false (参考).因此，默认 Nullable 实例的 HasValue 属性为 false；这反过来又使 Nullable<T> 实例本身的行为类似于 null.

• int? is syntactic sugar for the type Nullable<T> (where T is int), a struct. (reference)
• The Nullable<T> type has a bool HasValue member, which when false, makes the Nullable<T> instance "act like" a null value. In particular, the Nullable<T>.Equals method is overridden to return true when a Nullable<T> with HasValue == false is compared with an actual null value.
• From the C# Language Specification 11.3.4, a struct instance's initial default value is all of that struct's value type fields set to their default value, and all of that struct's reference type fields set to null.
• The default value of a bool variable in C# is false (reference). Therefore, the HasValue property of a default Nullable<T> instance is false; which in turn makes that Nullable<T> instance itself act like null.

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