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?? and ??= operators - the null-coalescing operators

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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. 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.

C# 
List<int>? numbers = null;
int? a = null;

Console.WriteLine((numbers is null)); // expected: true
// if numbers is null, initialize it. Then, add 5 to numbers
(numbers ??= new List<int>()).Add(5);
Console.WriteLine(string.Join(" ", numbers));  // output: 5
Console.WriteLine((numbers is null)); // expected: false        


Console.WriteLine((a is null)); // expected: true
Console.WriteLine((a ?? 3)); // expected: 3 since a is still null 
// if a is null then assign 0 to a and add a to the list
numbers.Add(a ??= 0);
Console.WriteLine((a is null)); // expected: false        
Console.WriteLine(string.Join(" ", numbers));  // output: 5 0
Console.WriteLine(a);  // output: 0

The left-hand operand of the ??= operator must be a variable, a property, or an indexer element.

The type of the left-hand operand of the ?? and ??= operators can't be a non-nullable value type. In particular, you can use the null-coalescing operators with unconstrained type parameters:

C# 
private static void Display<T>(T a, T backup)
{
    Console.WriteLine(a ?? backup);
}

The null-coalescing operators are right-associative. That is, expressions of the form

C# 
a ?? b ?? c
d ??= e ??= f

are evaluated as

C# 
a ?? (b ?? c)
d ??= (e ??= f)

Examples

The ?? and ??= operators can be useful in the following scenarios:

  • In expressions with the null-conditional operators ?. and ?[], you can use the ?? operator to provide an alternative expression to evaluate in case the result of the expression with null-conditional operations is null:

    C# 
    double SumNumbers(List<double[]> setsOfNumbers, int indexOfSetToSum)
{
    return setsOfNumbers?[indexOfSetToSum]?.Sum() ?? double.NaN;
}

var sum = SumNumbers(null, 0);
Console.WriteLine(sum);  // output: NaN

  • When you work with nullable value types and need to provide a value of an underlying value type, use the ?? operator to specify the value to provide in case a nullable type value is null:

    C# 
    int? a = null;
int b = a ?? -1;
Console.WriteLine(b);  // output: -1

    Use the Nullable<T>.GetValueOrDefault() method if the value to be used when a nullable type value is null should be the default value of the underlying value type.

  • You can use a throw expression as the right-hand operand of the ?? operator to make the argument-checking code more concise:

    C# 
    public string Name
{
    get => name;
    set => name = value ?? throw new ArgumentNullException(nameof(value), "Name cannot be null");
}

    The preceding example also demonstrates how to use expression-bodied members to define a property.

  • You can use the ??= operator to replace the code of the form

    C# 
    if (variable is null)
{
    variable = expression;
}

    with the following code:

    C# 
    variable ??= expression;

Operator overloadability

The operators ?? and ??= can't be overloaded.

C# language specification

For more information about the ?? operator, see The null coalescing operator section of the C# language specification.

For more information about the ??= operator, see the feature proposal note.

See also