试图了解?C#中的(空条件)运算符 [英] Trying to understand ?. (null-conditional) operator in C#

问题描述

我有一个非常简单的示例:

I have this very simple example:

class Program
{
    class A
    {
        public bool B;
    }

    static void Main()
    {
        System.Collections.ArrayList list = null;

        if (list?.Count > 0)
        {
            System.Console.WriteLine("Contains elements");
        }

        A a = null;

        if (a?.B)
        {
            System.Console.WriteLine("Is initialized");
        }
    }
}

if(list?.Count> 0)行可以完美编译，这意味着如果 list 为 null ，则表达式计数>默认情况下，0 变为 false .

The line if (list?.Count > 0) compiles perfectly which means that if list is null, the expression Count > 0 becomes false by default.

但是，行 if(a?.B)引发编译器错误，提示我无法将 bool?隐式转换为 bool

However, the line if (a?.B) throws a compiler error saying I can't implicitly convert bool? to bool.

为什么一个与另一个不同?

Why is one different from the other?

推荐答案

• list?.Count>0 :在这里，您将 int?与 int 进行比较，产生 bool ，因为
  • list?.Count > 0: Here you compare an int? to an int, yielding a bool, since lifted comparison operators return a bool, not a bool?.
  • a?.B: Here, you have a bool?. if, however, requires a bool.

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