Null coalescing assignment
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 (ยง11.18.3), 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 toA
orA0
(ifA0
exists), a compile-time error occurs. - If
A0
exists andB
is implicitly convertible toA0
, andB
is not dynamic, then the type ofa ??= b
isA0
.a ??= b
is evaluated at runtime as:
Except thatvar tmp = a.GetValueOrDefault(); if (!a.HasValue) { tmp = b; a = tmp; } tmp
a
is only evaluated once. - Otherwise, the type of
a ??= b
isA
.a ??= b
is evaluated at runtime asa ?? (a = b)
, except thata
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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