在 C# 中,两个问号一起意味着什么?

What do two question marks together mean in C#?(在 C# 中,两个问号一起意味着什么?)

本文介绍了在 C# 中,两个问号一起意味着什么?的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!

问题描述

Ran across this line of code:

FormsAuth = formsAuth ?? new FormsAuthenticationWrapper();

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

解决方案

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

FormsAuth = formsAuth ?? new FormsAuthenticationWrapper();

expands to:

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

which further expands to:

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

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

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;


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.)

这篇关于在 C# 中,两个问号一起意味着什么?的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持编程学习网!

本文标题为:在 C# 中,两个问号一起意味着什么?

基础教程推荐