The ?? Operator

Introduction

Scenario

Suppose you're assigning a value to Nullable bool like:

bool? b = null;

if(b) //Error CS0266.
{
 
}

So it's always preferable to use ?? to prevent error like:

if(b ?? false)
{
}

It defines that, in case b is null, pick the value false.

?? can also be used in multiple choice of value like:

bool ? a = null
bool ? b = null
bool ? c = true
 
a = b ?? c ?? false;

That will check b first if b is undefined or null, then it will move further to check for c if that also has null then it will set false to a.