Proposal: ??= operator

[manixrock]

Null-coalescing assignment operator, or fallback operator.

Allows this:

myNullableVariable = myNullableVariable ?? c;

To be re-written as this:

myNullableVariable ??= c;

It would be quite useful for lazy-initialized properties:

Person LoveMatch => mLoveMatchCache ?? mLoveMatchCache = Suitors.First(s => ComplexMatchingAlgorithm(s));
private Person mLoveMatchCache;

To become easier to write and understand:

Person LoveMatch => mLoveMatchCache ??= Suitors.First(s => ComplexMatchingAlgorithm(s));
private Person mLoveMatchCache;

It would also allow for variable fallbacks:

Person FindMatch(Person person, IEnumerable<Person> friends = null) {
    friends ??= FindFriends(person);
    // ...
}

It would work much like the other assignment operators. Of course, the right-hand side of the operation would only be executed if the left-hand side is null.

[HaloFour]

Dupe of (already closed as wont-fix) #205