What is the C# Null Coalescing Operator (??) and How Does it Work?

The empty merger operation symbol in the c#: the mystery of the dual question mark

In the field of C# programming, you may encounter a special syntax containing two question marks ("??"). This mysterious symbol is called an air merger computing, which is worth studying carefully.

Air merging operational symbol: Overview

The function of the air merger (represented by "??" is similar to the three yuan (immediately if) operator. It allows you to simply specify the reserve value used on the left expression calculation result.

Context examples

Consider the following code fragment:

Here, the empty merger computing character is used to set the value of FormsAnce variable to formsAuth (if it is not empty) or a new FormsAuthenticationWrapper instance (if Formsauth is empty).

The conditional statement of the extension is equivalent

FormsAuth = formsAuth ?? new FormsAuthenticationWrapper();

In order to better understand the behavior of the air merger operator, we can expand it into an equivalent conditional statement:

<一> Further simplify

This condition statement can be further written:

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

Therefore, the air merger computing character is essentially a shortcut to the if-Art S statement, which provides a simple way to specify the backup value.

<联> Class joint air merger operation symbol

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

You can link multiple air merger computing to check the empty value in turn. For example:

This statement assigns the first non -empty ANSWER# to ANSWER variable. If all ANSWER# values ​​are empty, the ANSWER variable is kept empty. <能> Performance precautions

It should be noted that the air merger operator only evaluates its expression once, whether they are true or false. This may be beneficial when one of the expressions has side effects.

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

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