null-conditional await

Proposed
Prototype: None
Implementation: None
Specification: Started, below

Summary

Support an expression of the form await? e, which awaits e if it is non-null, otherwise it results in null.

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

We add a new form of the await_expression:

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.

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

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

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

Requires LDM review

Design meetings

None.

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