问题描述

为什么 C# 编译器甚至不抱怨此代码的警告?:

Why does the C# compiler not even complain with a warning on this code? :

if (this == null)
{
   // ...
}

显然条件将永远被满足..

推荐答案

因为你可以 覆盖 operator == 以在这种情况下返回 true.

Because you could override operator == to return true for that case.

public class Foo
{
    public void Test()
    {
        Console.WriteLine(this == null);
    }

    public static bool operator ==(Foo a, Foo b)
    {
        return true;
    }

    public static bool operator !=(Foo a, Foo b)
    {
        return true;
    }
}

运行 new Foo().Test() 会在控制台打印True".

Running new Foo().Test() will print "True" to the console.

这里的另一个问题是:为什么编译器不对 ReferenceEquals(this, null) 发出警告?从上面链接的底部:

The other question here is: why doesn't the compiler issue a warning for ReferenceEquals(this, null)? From the bottom of the above link:

operator == 重载的一个常见错误是使用 (a == b), (a == null),或 (b == null) 来检查引用的相等性.这反而会导致调用重载的 operator ==，从而导致无限循环.使用 ReferenceEquals 或将类型强制转换为 Object，以避免循环.

A common error in overloads of operator == is to use (a == b), (a == null), or (b == null) to check for reference equality. This instead results in a call to the overloaded operator ==, causing an infinite loop. Use ReferenceEquals or cast the type to Object, to avoid the loop.

那个可能会在@Aaronaught 的回复中得到解答.这也是为什么你应该做 (object)x == null 或 ReferenceEquals(x, null)，而不是做一个简单的 x == null代码>，当您检查空引用时.当然，除非您确定 == 运算符没有重载.

That might be answered by @Aaronaught's response. And that's also why you should be doing (object)x == null or ReferenceEquals(x, null), not doing a simple x == null, when you're checking for null references. Unless, of course, you're sure that the == operator is not overloaded.

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