What is the operator precedence of C# null-coalescing (??) operator?

Question

I've just tried the following, the idea being to concatenate the two strings, substituting an empty string for nulls.

string a="Hello";
string b=" World";

-- Debug (amusing that ? is print, doesn't exactly help readability...)

 ? a ?? "" + b ?? "" 

-> "Hello"

Correct is:

? (a??"")+(b??"")
"Hello World"

I was kind of expecting "Hello World", or just "World" if a is null. Obviously this is todo with operator precedence and can be overcome by brackets, is there anywhere that documents the order of precedence for this new operator.

(Realising that I should probably be using stringbuilder or String.Concat)

Thanks.

Answer 1

Aside from what you'd like the precedence to be, what it is according to ECMA, what it is according to the MS spec and what csc actually does, I have one bit of advice:

Don't do this.

I think it's much clearer to write:

string c = (a ?? "") + (b ?? "");

Alternatively, given that null in string concatenation ends up just being an empty string anyway, just write:

string c = a + b;

EDIT: Regarding the documented precedence, in both the C# 3.0 spec (Word document) and ECMA-334, addition binds tighter than ??, which binds tighter than assignment. The MSDN link given in another answer is just wrong and bizarre, IMO. There's a change shown on the page made in July 2008 which moved the conditional operator - but apparently incorrectly!