?? and ??= operators - the null-coalescing 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. 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.
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:
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
a ?? b ?? c
d ??= e ??= f
are evaluated as
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 isnull
: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 isnull
: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: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 formif (variable is null) { variable = expression; }
with the following code:
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.