null-coalescing operator evaluation?

Question

object bread(food foo)
{
    return foo.ingredient ?? "cheese";
}

If foo exits, but ingredient is null, I get "cheese". My question, including an assumption:
If foo itself is null will "chesse" be returned or will it throw an ArgutmentNullException?

My GUESS is that the NullCoalescingOperator is implemented more or less like this:

object nco(object lhs, object rhs)
{
    if(lhs != null)
        return lhs;
    else
        return rhs;
}

Therefore passing foo.ingredient already causes the exception(since you cannot check a field in an object that you don't have) and thus it is thrown.
Would make sense.

Is this idea correct / how is the nco implemented and why?

Do you mean `??` or `?`? What C# version? I have read about a syntax like `foo?.ingredient` but am not sure this made it into C# yet. — CodeCaster, Jul 09 '14 at 11:15
You can assert your own assumption by trying your bread example in a simple test project. What is the reason you didn't do that? — MarkO, Jul 09 '14 at 11:15
@MarkO quite practically the reason is, that I'm currently on the train, when I started to wonder about this. — Mark, Jul 09 '14 at 11:18
Note that in [some versions of C#](https://roslyn.codeplex.com/wikipage?title=Language%20Feature%20Status&referringTitle=Documentation) it will be allowed to use `return foo?.ingredient ?? "cheese";` where the operator `?.` is a new one in C#. See my link. — Jeppe Stig Nielsen, Jul 09 '14 at 11:21
@Jeppe thanks, "null propagation" is the term I couldn't think of. See for example this (badly-titled) question: [Possible pitfalls of using this (extension method based) shorthand](http://stackoverflow.com/questions/123088/possible-pitfalls-of-using-this-extension-method-based-shorthand). — CodeCaster, Jul 09 '14 at 11:23
@JeppeStigNielsen cutting edge knowledge right there. That'll be an interesting option in the future. — Mark, Jul 09 '14 at 11:25

score 5 · Accepted Answer · edited Jul 09 '14 at 11:25

5

You will get a NullReferenceException if foo is null.

So you've got to treat this case, with a ternary (if else) rather than a coalesce.

return (foo == null || foo.ingredient == null) 
           ? "cheese" 
           : foo.ingredient;

edited Jul 09 '14 at 11:25

Nikhil Agrawal

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answered Jul 09 '14 at 11:16

Raphaël Althaus

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Answer plus a way to achive the intended and I didn't even ask for that, Sweet. – Mark Jul 09 '14 at 11:29

score 3 · Answer 2 · answered Jul 09 '14 at 11:21

Yes, you idea is absolutely correct because:

foo.ingredient ?? "cheese";

Equals to:

foo.ingredient != null ? foo.ingredient : "cheese";

What you might like, in VS2014 CTP they already have new operator ?., which will do what you need:

foo?.ingredient ?? "cheese";

Tarec · Answer 3 · 2014-07-09T12:04:39.463

1

Of course it'll throw an exception. You cannot access a property of the object, that's not there. If you want secure yourself from an error, you could write:

return (foo == null) ? "cheese" : (foo.ingredient ?? "cheese");

edited Jul 09 '14 at 12:04

answered Jul 09 '14 at 11:19

Tarec

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I can see Raphaël Althaus's answer is even cleaner. – Tarec Jul 09 '14 at 11:20

null-coalescing operator evaluation?

3 Answers3