Null coalescing operator

The null coalescing operator is a binary operator that is part of the syntax for a basic conditional expression in several programming languages, such as (in alphabetical order): C#^[1] since version 2.0,^[2] Dart^[3] since version 1.12.0,^[4] PHP since version 7.0.0,^[5] Perl since version 5.10 as logical defined-or,^[6] PowerShell since 7.0.0,^[7] and Swift^[8] as nil-coalescing operator. It is most commonly written as x ?? y, but varies across programming languages.

While its behavior differs between implementations, the null coalescing operator generally returns the result of its left-most operand if it exists and is not null, and otherwise returns the right-most operand. This behavior allows a default value to be defined for cases where a more specific value is not available.

Like the binary Elvis operator, usually written as x ?: y, the null coalescing operator is a short-circuiting operator and thus does not evaluate the second operand if its value is not used, which is significant if its evaluation has side-effects.