C# Variable Name "_" (underscore) only

Question

I was just hit with a minor issue in C#, it was just a copy-paste mistake but don't know how C# accept it.

This code gets compiled successfully...HOW

namespace DemoNS
{
    class DemoClass
    {
        String _ = new String('a', 1);        
    }
}

Is there any default significance of variable named _?

Answer 1

Nowadays with C# 7.0 the _ does have significance sometimes. It became the discard operator for the new out var feature. It is used when a function returns a value and you want to notify the compiler that you won't be using it - so it can be optimized out. Or when deconstructing (Another C# 7.0 feature) you can use it to ignore part of the tuple that you are not interested in.

Example out var

void Test(out int i) => i = 1;

Test(out _); // _ was never declared, it will still compile in C# 7.0

var r = _;   // error CS0103: The name '_' does not exist in the current context

Example deconstructing a Tuple

var Person = ("John", "Smith");

var (First, _) = Person; // '_' is not a declared

Debug.Print(First); // prints "John"
Debug.Print(_); // error CS0103: The name '_' does not exist in the current context

A problem arises if you do declare your own variable named _ and then use the discard operator it will cause ambiguity. This issue has been reported Here.

EDIT Above problem is not a problem as @maf-soft points out in comments. If _ was declared it is treated like a regular variable like it was pre C# 7.0.

EDIT 2021 A little overdue

In c# 8.0 _ also became the catch all operator in a switch expression, officially named the discard operator

Example discard operator

var moreThan20 = val switch
{
    >20 => "Yes",
    >50 => "Yes - way more!",
    _ => "No",
};

The discard operator assigns a value when no other pattern matches