测试与 NSNull 的相等性 [英] Testing equality to NSNull

问题描述

下面是一个代码块，用于测试字典是否为空，如果不是，则拉出正确的对象.然而，出于某种原因，尽管 if 检查失败，代码仍然执行.NSNull 的工作方式是否有一些我不理解的怪癖，或者这是 Apple 的错误?

Below is a code block, that is supposed to test to see if a dictionary is null, and if it isn't, pull out the correct object. However, for some reason, despite the fact that the if check fails, the code still executes. Is there some quirk with how NSNull works that I don't understand, or is this an Apple bug?

if (svcUser && !(svcUser == (id)[NSNull null])) {
    return [svcUser objectForKey:@"access_level"];
}

控制台响应:

(lldb) print svcUser && !(svcUser == (id)[NSNull null])
(bool) $0 = false
(lldb) continue
-[NSNull objectForKey:]: unrecognized selector sent to instance 0x2b51678

推荐答案

只需检查:

svcUser == [NSNull null]

这是 Apple 在其 文档.

This is the approach that Apple mentions in their docs.

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