Coalescing operator with default nullable struct is treated as default(struct)

[bollhals]

Hi

I had a strange finding today that caused me to break some test cases of mine, I was unware of this and I was wondering whether this is by intention.

So basically if you do this

Console.WriteLine(((int?)null) ?? default)

it will write "0", as the default is default(int) and not default(int?).

Is this a bug or what am I missing here?

Thanks

[svick]

This is the expected behavior according to the spec (edited for brevity):

In order of preference, the type of a ?? b is A₀, A, or B, where A is the type of a (provided that a has a type), B is the type of b (provided that b has a type), and A₀ is the underlying type of A if A is a nullable value type, or A otherwise. Specifically, a ?? b is processed as follows:

• […]
• If A is a nullable value type and an implicit conversion exists from b to A₀, the result type is A₀. At run-time, a is first evaluated. If a is not null, a is unwrapped to type A₀, and this becomes the result. Otherwise, b is evaluated and converted to type A₀, and this becomes the result.
• […]

In this case, A is int? and A₀ is int. And since an implicit conversion exists from default to int, the result is (int)default.

[bollhals]

Thanks for the reply!

What is B (default) in my example? int or int?

if B is int, then B equals A₀
if B is int?, then B does not equal A₀, but also can not be implictly converted to int.

So then the question is, why is default in the above case int and not int?

Or is default without more type info matching the A₀ type and not the A type, thus being 0?

[svick]

What is B (default) in my example? int or int?

As I understand it, the default literal by itself does not have a type, so B does not exist (I have previously edited the parentheticals describing that case out, I guess I shouldn't have). But it can be implicitly converted to any type.

Notice that B is not actually used in the bullet point that applies here, so it's okay that it does not exist.

Or is default without more type info matching the A₀ type and not the A type, thus being 0?

Like the spec says, the order of preference is A₀, A, B. And since A₀ works, it's the type that's chosen.

[jnm2]

I also want to point out that ?? default is a convenient thing to be able to do with Nullable<T> values and have it mean something different from ?? null. It is idiomatic C# and it compiles into .GetValueOrDefault().