C#7: Underscore ( _ ) & Star ( * ) in Out variable

Question

I was reading about new out variable features in C#7 here. I have two questions:

1. It says

   We allow "discards" as out parameters as well, in the form of a _, to let you ignore out parameters you don’t care about:

   p.GetCoordinates(out var x, out _); // I only care about x 

   Q: I guess this is just an info and not a new feature of C#7 because we can do so in pre C#7.0 too:

   var _; if (Int.TryParse(str, out _)) ... 

   or am I missing something here?

2. My code gives an error when I do as mentioned in same blog:

   ~Person() => names.TryRemove(id, out *); 

   * is not a valid identifier. An oversight by Mads Torgersen I guess?

Answer 1

Discards, in C#7 can be used wherever a variable is declared, to - as the name suggests - discard the result. So a discard can be used with out variables:

p.GetCoordinates(out var x, out _); 

and it can be used to discard an expression result:

_ = 42; 

In the example,

p.GetCoordinates(out var x, out _); _ = 42; 

There is no variable, _, being introduced. There are just two cases of a discard being used.

If however, an identifier _ exists in the scope, then discards cannot be used:

var _ = 42; _ = "hello"; // error - a string cannot explicitly convert from string to int 

The exception to this is when a _ variable is used as an out variable. In this case, the compiler ignores the type or var and treats it as a discard:

if (p.GetCoordinates(out double x, out double _)) {     _ = "hello"; // works fine.     Console.WriteLine(_); // error: _ doesn't exist in this context. } 

Note that this only occurs if, in this case, out var _ or out double _ is used. Just use out _ and then it's treated as a reference to an existing variable, _, if it's in scope, eg:

string _; int.TryParse("1", out _); // complains _ is of the wrong type 

Finally, the * notation was proposed early in the discussions around discards, but was abandoned in favour of _ due to the latter being a more commonly used notation in other languages.

Answer 2

Another example of the Discard Operator _ in C# 7 is to pattern match a variable of type object in a switch statement, which was recently added in C# 7:

Code:

static void Main(string[] args) {     object x = 6.4;      switch (x)     {         case string _:             Console.WriteLine("it is string");             break;         case double _:             Console.WriteLine("it is double");             break;         case int _:             Console.WriteLine("it is int");             break;         default:             Console.WriteLine("it is Unknown type");             break;     }      // end of main method } 

This code will match the type and discard the variable passed to the case ... _.