# null coalescing assignment * [x] Proposed * [ ] Prototype: Not Started * [ ] Implementation: Not Started * [ ] Specification: Below ## Summary [summary]: #summary Simplifies a common coding pattern where a variable is assigned a value if it is null. As part of this proposal, we will also loosen the type requirements on `??` to allow an expression whose type is an unconstrained type parameter to be used on the left-hand side. ## Motivation [motivation]: #motivation It is common to see code of the form ```csharp if (variable == null) { variable = expression; } ``` This proposal adds a non-overloadable binary operator to the language that performs this function. There have been at least eight separate community requests for this feature. ## Detailed design [design]: #detailed-design We add a new form of assignment operator ``` antlr assignment_operator : '??=' ; ``` Which follows the [existing semantic rules for compound assignment operators](../../spec/expressions.md#compound-assignment), except that we elide the assignment if the left-hand side is non-null. The rules for this feature are as follows. Given `a ??= b`, where `A` is the type of `a`, `B` is the type of `b`, and `A0` is the underlying type of `A` if `A` is a nullable value type: 1. If `A` does not exist or is a non-nullable value type, a compile-time error occurs. 2. If `B` is not implicitly convertible to `A` or `A0` (if `A0` exists), a compile-time error occurs. 3. If `A0` exists and `B` is implicitly convertible to `A0`, and `B` is not dynamic, then the type of `a ??= b` is `A0`. `a ??= b` is evaluated at runtime as: ```C# var tmp = a.GetValueOrDefault(); if (!a.HasValue) { tmp = b; a = tmp; } tmp ``` Except that `a` is only evaluated once. 4. Otherwise, the type of `a ??= b` is `A`. `a ??= b` is evaluated at runtime as `a ?? (a = b)`, except that `a` is only evaluated once. For the relaxation of the type requirements of `??`, we update the spec where it currently states that, given `a ?? b`, where `A` is the type of `a`: > 1. If A exists and is not a nullable type or a reference type, a compile-time error occurs. We relax this requirement to: 1. If A exists and is a non-nullable value type, a compile-time error occurs. This allows the null coalescing operator to work on unconstrained type parameters, as the unconstrained type parameter T exists, is not a nullable type, and is not a reference type. ## Drawbacks [drawbacks]: #drawbacks As with any language feature, we must question whether the additional complexity to the language is repaid in the additional clarity offered to the body of C# programs that would benefit from the feature. ## Alternatives [alternatives]: #alternatives The programmer can write `(x = x ?? y)`, `if (x == null) x = y;`, or `x ?? (x = y)` by hand. ## Unresolved questions [unresolved]: #unresolved-questions - [ ] Requires LDM review - [ ] Should we also support `&&=` and `||=` operators? ## Design meetings None.