78 lines
2.4 KiB
Markdown
78 lines
2.4 KiB
Markdown
# 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
|
|
|
|
``` c#
|
|
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](https://github.com/dotnet/csharplang/blob/master/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`:
|
|
|
|
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`, a compile-time error occurs.
|
|
3. The type of `a ??= b` is `A`.
|
|
4. `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.
|
|
|
|
## 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.
|