[Proposal] Null-coalescing Assignment Operator: ??=

??= would have semantics analogous to the existing compound assignment operators like +=, %=, and <<=. More precisely, x ??= y would be equivalent to x = x ?? y except that x is evaluated only once. It would have clear, if not vast, utility. Hopefully, it would not be an unreasonably large effort.

It is perhaps debatable whether

object o = GetFirstChoice( foo, bar, someLongArgument )
         ?? GetSecondChoice( foo, bar, someLongArgument )
         ?? GetFallback();

looks better than

object o = GetFirstChoice( foo, bar, someLongArgument );
o ??= GetSecondChoice( foo, bar, someLongArgument );
o ??= GetFallback();

However, the latter seems easier to step through in the debugger.

Furthermore, a use case like

private static void ApplyNewSettings( SettingsSource newSource ) {
    // Set each setting if and only if it is not already set to a non-null value.
    // This could be e.g. applying defaults after deserializing user settings.
    this.SomeSetting ??= newSource.SomeSetting;
    // ...
}

is less repetitive with ??=.

[ashmind]

Given the amount of interest in it (5 linked items) I’ve decided to try making a prototype – which is currently at https://github.com/ashmind/roslyn/tree/features/coalesce-assignment. That’s my first attempt at a Roslyn thing, so I’m sure I’m doing everything wrong.

I’ve also added that branch to TryRoslyn: http://tryroslyn.azurewebsites.net/#b:ashmind-features-coalesce-assignment/K4Zwlgdg5gBAygTxAFwKYFsDcAoADsAIwBswBjGUogQxBBgGEYBvbGNmFAJ0lgA8d2HZN2gwEA9q3b5iZGADcA9mAAmMALIAKAJTMpgtrxgBeGACJeZiQZgB6WzAAWyZLhAAue1DDJHhAHSkiui2KorIEKjItpyKIEQIELZgtMCoILYATAAMAKz6BggwAPzFpvwFAL7YlUA=

[gafter]

The benefits of this proposal do not overcome the drawbacks in terms of language complexity. It is unlikely to ever have a high enough benefit-for-cost to make us prefer it over many other things we could do in this release or many releases into the future.