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What is Double Question mark operator (??) in C#

A not so common, but very useful operator is the double question mark operator (??). This can be very useful while working with nullable types.
 
Let’s say you have two nullable int:
  1. int? numOne = null;  
  2. int? numTwo = 23;  
Scenario: If numOne has a value, you want it, if not you want the value from numTwo, and if both are null you want the number ten (10).
 
Old solution:
  1. {  
  2.    if (numOne != null)  
  3.    {  
  4.       return numOne;  
  5.    }  
  6.    if (numTwo != null)  
  7.    {  
  8.       return numTwo;  
  9.    }  
  10.    return 10;  
  11. }  
Or another old solution with a single question mark:
  1. { return numOne != null ? numOne : (numTwo != null ? numTwo : 10); }  
But with the double question mark operator we can do this:
  1. { return (numOne ?? numTwo) ?? 10; }  
As you can see, the double question mark operator returns the first value that is not null. It is more concise.