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.

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

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, 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:

If A does not exist or is a non-nullable value type, a compile-time error occurs.
If B is not implicitly convertible to A or A0 (if A0 exists), a compile-time error occurs.
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:
```
var tmp = a.GetValueOrDefault();
if (!a.HasValue) { tmp = b; a = tmp; }
tmp
```
Except that a is only evaluated once.
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:

If A exists and is not a nullable type or a reference type, a compile-time error occurs.

We relax this requirement to:

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

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), if (x == null) x = y;, or x ?? (x = y) by hand.

Unresolved questions

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

Design meetings

None.

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