C# - ?? and ??= operators (Null-Coalescing)

?? and ??= operators

The null-coalescing operator ?? returns the value of its left-hand operand if it isn't null; otherwise, it evaluates the right-hand operand and returns its result. The ?? operator doesn't evaluate its right-hand operand if the left-hand operand evaluates to non-null.

Available in C# 8.0 and later, the null-coalescing assignment operator ??= assigns the value of its right-hand operand to its left-hand operand only if the left-hand operand evaluates to null. The ??= operator doesn't evaluate its right-hand operand if the left-hand operand evaluates to non-null.

Example 1:

 List<int> numbers = null;

 int? a = null;

 (numbers ??= new List<int>()).Add(5);

 Console.WriteLine(string.Join(" ", numbers));  // output: 5

 numbers.Add(a ??= 0);

 Console.WriteLine(string.Join(" ", numbers));  // output: 5 0

 Console.WriteLine(a);  // output: 0

Example 2:

 double SumNumbers(List<double[]> setsOfNumbers, int indexOfSetToSum)

 {

     return setsOfNumbers?[indexOfSetToSum]?.Sum() ?? double.NaN;

 }

 var sum = SumNumbers(null, 0);

 Console.WriteLine(sum);  // output: NaN

Example 3:

public string Name

{

get => name;

set => name = value ??

throw new ArgumentNullException(nameof(value), "Name cannot be null");

}