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!