out var _ and out _ difference?

Question

In C# 7 we can do like this:

byte.TryParse(p.Value, out _)

or like this

byte.TryParse(p.Value, out var _)

Are there any differences?

Answer 1

The difference is how they handle an existing variable named _.

string _;

int.TryParse("123", out var _); // legal syntax for discard
int.TryParse("123", out _); // compile error, string _ is incompatible with out int parameter

or

int _;

int.TryParse("123", out var _); // discard
Console.WriteLine(_); // error: variable is not assigned
int.TryParse("123", out _); // passes existing variable byref
Console.WriteLine(_); // ok: prints "123"

The reason for this is that out _ already had a meaning, and the language maintains backward compatibility with that old meaning. But out typename identifier is new syntax in C# 7, so there's no backward compatibility constraint.

Answer 2

As opposed to what others in their comments said: No, there are no differences. They both produce the exact same IL.

Both

byte.TryParse(p.Value, out _);
Console.WriteLine(_);

and

byte.TryParse(p.Value, out var _);
Console.WriteLine(_);

will produce a compiler error with C#7, since _ is not intended to be used.

The usage of _ is not restricted to out parameters, but can be used for returns, too (as Evk pointed out)

byte.TryParse(p.Value, out var _); // I don't care about the out variable
_ = SomeMethod(); // I don't care about the return value

There is an excellent answer covering most things about ommitted parameters here.

Remarks: I would prefer out _ over out var _, since there is a clear syntactic distinction between out _ and out var legalVariableName.

EDIT

Obviously I'm not quite right here. There are some subtle differences, see Ben Voigts answer.