C# – Operator Precedence of Null-Coalescing (??) Operator

c++nullnull-coalescing-operator

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.

Best Answer

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!

Related Solutions

C# Null Coalescing Operator – Understanding Weird Operator Precedence with ??

The null coalescing operator has very low precedence so your code is being interpreted as:

int? x = 10;
string s = ("foo" + x) ?? (0 + "bar");

In this example both expressions are strings so it compiles, but doesn't do what you want. In your next example the left side of the ?? operator is a string, but the right hand side is an integer so it doesn't compile:

int? x = 10;
string s = ("foo" + x) ?? (0 + 12);
// Error: Operator '??' cannot be applied to operands of type 'string' and 'int'

The solution of course is to add parentheses:

int? x = 10;
string s = "foo" + (x ?? 0) + "bar";

C# – Understanding the Null Coalescing Operator (??)

(This answer was constructed from my comments above.)

Are you sure the debugger dsiplays this correctly to you? Have you tried stepping some lines further down to make sure you have the updated value of amount3?

I'm sure it's just an issue with the debugger. Sometimes you have to step a little further. Maybe the translated code (IL) has some optimizations that confuse the debugger (or what would I know). But without the debugger, the value will be updated exactly when you expect it.

I've seen other experienced developers being confused by similar situations, so I know the debugger sometimes is "one line of code" behind when looking at an assignment to a local variable. Maybe someone can find a link discussing that?

Best Answer

Related Solutions

C# Null Coalescing Operator – Understanding Weird Operator Precedence with ??

C# – Understanding the Null Coalescing Operator (??)

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