C# 可以将值类型与 null 进行比较

问题描述

我今天遇到了这个问题，不知道为什么 C# 编译器没有抛出错误.

I ran into this today and have no idea why the C# compiler isn't throwing an error.

Int32 x = 1;
if (x == null)
{
    Console.WriteLine("What the?");
}

我很困惑 x 怎么可能为空.特别是因为这个赋值肯定会引发编译器错误:

I'm confused as to how x could ever possibly be null. Especially since this assignment definitely throws a compiler error:

Int32 x = null;

x 是否有可能变为空值，是微软决定不把这个检查放入编译器，还是完全错过了?

Is it possible that x could become null, did Microsoft just decide to not put this check into the compiler, or was it missed completely?

更新:在弄乱了写这篇文章的代码后，编译器突然提出警告，该表达式永远不会为真.现在我真的迷路了.我将对象放入一个类中，现在警告消失了，但留下了一个问题，值类型最终是否可以为空.

Update: After messing with the code to write this article, suddenly the compiler came up with a warning that the expression would never be true. Now I'm really lost. I put the object into a class and now the warning has gone away but left with the question, can a value type end up being null.

public class Test
{
    public DateTime ADate = DateTime.Now;

    public Test ()
    {
        Test test = new Test();
        if (test.ADate == null)
        {
            Console.WriteLine("What the?");
        }
    }
}

推荐答案

这是合法的，因为运算符重载解析有唯一的最佳运算符可供选择.有一个 == 运算符，它接受两个可为空的整数.int local 可转换为可为空的 int.null 文字可转换为可为 null 的 int.因此，这是 == 运算符的合法用法，并且总是会导致错误.

This is legal because operator overload resolution has a unique best operator to choose. There is an == operator that takes two nullable ints. The int local is convertible to a nullable int. The null literal is convertible to a nullable int. Therefore this is a legal usage of the == operator, and will always result in false.

同样，我们也允许你说if (x == 12.6)"，这也总是假的.int local 可转换为 double，文字可转换为 double，显然它们永远不会相等.

Similarly, we also allow you to say "if (x == 12.6)", which will also always be false. The int local is convertible to a double, the literal is convertible to a double, and obviously they will never be equal.