55 lines
1.8 KiB
Markdown
55 lines
1.8 KiB
Markdown
# null-conditional await
|
|
|
|
* [x] Proposed
|
|
* [ ] Prototype: None
|
|
* [ ] Implementation: None
|
|
* [ ] Specification: Started, below
|
|
|
|
## Summary
|
|
[summary]: #summary
|
|
|
|
Support an expression of the form `await? e`, which awaits `e` if it is non-null, otherwise it results in `null`.
|
|
|
|
## Motivation
|
|
[motivation]: #motivation
|
|
|
|
This is a common coding pattern, and this feature would have nice synergy with the existing null-propagating and null-coalescing operators.
|
|
|
|
## Detailed design
|
|
[design]: #detailed-design
|
|
|
|
We add a new form of the *await_expression*:
|
|
|
|
```antlr
|
|
await_expression
|
|
: 'await' '?' unary_expression
|
|
;
|
|
```
|
|
|
|
The null-conditional `await` operator awaits its operand only if that operand is non-null. Otherwise the result of applying the operator is null.
|
|
|
|
The type of the result is computed using the [rules for the null-conditional operator](https://github.com/dotnet/csharplang/blob/master/spec/expressions.md#null-conditional-operator).
|
|
|
|
> **NOTE:**
|
|
> If `e` is of type `Task`, then `await? e;` would do nothing if `e` is `null`, and await `e` if it is not `null`.
|
|
>
|
|
> If `e` is of type `Task<K>` where `K` is a value type, then `await? e` would yield a value of type `K?`.
|
|
|
|
## 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
|
|
|
|
Although it requires some boilerplate code, uses of this operator can often be replaced by an expression something like `(e == null) ? null : await e` or a statement like `if (e != null) await e`.
|
|
|
|
## Unresolved questions
|
|
[unresolved]: #unresolved-questions
|
|
|
|
- [ ] Requires LDM review
|
|
|
|
## Design meetings
|
|
|
|
None.
|