Validity of discard _ in csharp

As a valid identifier in C#, there's different scenarios for _ to behave as a regular reference or a true discard.

What might broke _?

When _ as a typed variable or member of a type, it's a true identifier

int _ = 1;
Console.WriteLine(_);
Console.WriteLine(Shared._); // valid

static class Shared 
{
    public static int _ = 1;
}

_ inside for deconstruction, parameter list and inline out can be typed, but they're all valid discards.

// Typed discards
(int _, var _) = (1, "1");
int.TryParse("1", out var _);
var f = (int _, string _) => Console.WriteLine(_); // error CS0103: The name '_' does not exist in the current context
Console.WriteLine(_); // error CS0103: The name '_' does not exist in the current context

// Untyped discards
_ = 1;
_ = "1";
(_, _) = (1, "1");
int.TryParse("1", out _);
// you can't have a untyped delegate...
Console.WriteLine(_); // error CS0103: The name '_' does not exist in the current context

However, untyped discards can be broke if there exists a true _ identifier.

int _ = 1;
(_, _) = ("1", 1); // Cannot assign string to int
int.TryParse("1", out _) // Assign to _, it's not a discard now

TIP

That's why we should prefer typed discard if we do need to be cautious about declared _

INFO

C# is going to make _ as discards everywhere? See: Open issues: Breaking changes

lambdas

Currently only when parameter list contains more than one discard can it be treated as true discard.

var f = (int _) => Console.WriteLine(_); // valid
var m = (int _, string _) => Console.WriteLine(_); // both discards