The ?? operator

Introduction

Operator is introduced with Nullable datatype inclusion in .NET Framework operator ?? can also be referred in a word like 'In case of null, pick value from another'

Scenario

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

bool? b = null;

At the time of checking value, it will give you an error like:

if(b) //Error CS0266.
{
 
}

So it's always preferable using ?? to prevent error like

if(b ?? false)
{
}

It defines that, in case b is null, pick the value false.
?? can be also 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's also having null then it will set false to a.