null coalescing assignment

Proposed
Prototype: Not Started
Implementation: Not Started
Specification: Below

Summary

Simplifies a common coding pattern where a variable is assigned a value if it is null.

Motivation

It is common to see code of the form

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

We add a new form of assignment operator

assignment_operator
    : '??='
    ;

Which follows the existing semantic rules for compound assignment operators. What that means is that an operation of the form x ??= y is processed as if written x = (x ?? y) (except x shall be computed once). However, we permit the assignment to x (of its existing value) to be elided in the case that x is not null.

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

The programmer can write (x = x ?? y) or if (x == null) x = y; by hand.

Unresolved questions

Requires LDM review
Should we also support &&= and ||= operators?

Design meetings

None.

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