Simplifies a common coding pattern where a variable is assigned a value if it is null.
## Motivation
[motivation]: #motivation
It is common to see code of the form
``` c#
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
[design]: #detailed-design
We add a new form of assignment operator
``` antlr
assignment_operator
: '??='
;
```
Which follows the [existing semantic rules for compound assignment operators](https://github.com/dotnet/csharplang/blob/master/spec/expressions.md#compound-assignment). 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
[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
The programmer can write `(x = x ?? y)` or `if (x == null) x = y;` by hand.
