C#中两个问号放在一起是什么意思? [英] What do two question marks together mean in C#?

问题描述

跑过这行代码:

FormsAuth = formsAuth ?? new FormsAuthenticationWrapper();

这两个问号是什么意思，是某种三元运算符吗?很难在 Google 中查找.

What do the two question marks mean, is it some kind of ternary operator? It's hard to look up in Google.

推荐答案

它是空合并运算符，非常类似于三元 (immediate-if) 运算符.另请参阅 ??运营商 - MSDN.

It's the null coalescing operator, and quite like the ternary (immediate-if) operator. See also ?? Operator - MSDN.

FormsAuth = formsAuth ?? new FormsAuthenticationWrapper();

扩展为:

FormsAuth = formsAuth != null ? formsAuth : new FormsAuthenticationWrapper();

进一步扩展为:

if(formsAuth != null)
    FormsAuth = formsAuth;
else
    FormsAuth = new FormsAuthenticationWrapper();

在英语中，它的意思是如果左边的东西不为空，就使用它，否则使用右边的东西."

In English, it means "If whatever is to the left is not null, use that, otherwise use what's to the right."

请注意，您可以按顺序使用任意数量的这些.以下语句将第一个非空 Answer# 分配给 Answer(如果所有 Answers 都为空，则 Answer 为空):

Note that you can use any number of these in sequence. The following statement will assign the first non-null Answer# to Answer (if all Answers are null then the Answer is null):

string Answer = Answer1 ?? Answer2 ?? Answer3 ?? Answer4;

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另外值得一提的是，虽然上面的扩展在概念上是等价的，但每个表达式的结果只计算一次.例如，如果表达式是具有副作用的方法调用，则这一点很重要.(感谢@Joey 指出这一点.)

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Also it's worth mentioning while the expansion above is conceptually equivalent, the result of each expression is only evaluated once. This is important if for example an expression is a method call with side effects. (Credit to @Joey for pointing this out.)

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