c# - 我怎么能在int上执行方法？在没有 NullReferenceException 的情况下设置为 null？

Question

我在 MSDN 上读到:

The null keyword is a literal that represents a null reference, one that does not refer to any object.

但是我看到下面的代码运行时没有抛出任何异常:

int? i = null;
var s = i.ToString();

那么如果变量i 为null，为什么我可以执行它的方法呢？

Answer 1

因为 int?实际上是一个 Nullable<Int32>和 Nullable<T> 是 struct , 和一个结构 cannot be null .

就是这样Nullable types work.它们不是引用值，所以它们不能为空，但当它们被认为等同于空时，它们可以有一个状态。

您可以获得有关Nullable<T> 的更多详细信息在 how are nullable types implemented under the hood in .net? 中实现和 Nullable<T> implementation

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虽然正如@JeppeStigNielsen 所指出的，但在一种情况下您可以获得 NRE:

However: When boxed to a reference type, special treatment of Nullable<> ensures we do get a true null reference. So for example i.GetType() with i as in the question will blow up with the NullReferenceException. That is because this method is defined on object and not overridable