Why can't I do ??= in C#?

Question

I often find myself doing:

foo = foo ?? x;

Why can't I do:

foo ??= x;

Edit: I know it's not part of the language... My question is "why not"? I find the necessity to repeat "foo" to be unpleasing and potentially error-prone. It looks just as ugly as:

foo = foo + x;

Answer 1

When I think about it,

foo = foo ?? x 

is really just

foo = foo != null ? foo : x

and at that point, the analogy to += starts to fall apart.

Answer 2

There's no reason why there couldn't be such an operator, but I suspect the added complexity of the language, though slight, outweights the benefit (which I would deem "very slight").

Basically any addition to the language has a pretty high bar to clear. You say you use this quite often - how often, really? While I deem ?? handy, I can't say I use even that terribly frequently.