On '@' and reserved words

[stephentoub]

From @juliusfriedman on July 19, 2016 16:28

Attempting to compile the following code:

static void @_(object someName)
        {
            object @someName = null;

            return;
        }

results in a compilation error:

Error 79 A local variable named 'someName' cannot be declared in this scope because it would give a different meaning to 'someName', which is already used in a 'parent or current' scope to denote something else.

Additionally it seems that the @ character is used as an Escape in some regard in the parser logic and thus it's appearance causes the character which follows to lose it's meaning as can be seen in the following example:

 static void @(object someName)
        {
            object @someName = null;

            return;
        }

Thanks for your time and consideration.

Copied from original issue: dotnet/corefx#10152

[svick]

This is the expected behavior. @ is meant to be used to allow you to use keywords as identifiers (e.g. if you want to create a class with name class, you have to write class @class). It's also possible to use it with normal identifiers, but then the @ is not considered part of the identifier.

This explains the behavior you see:

• someName and @someName are considered to be the same identifier, which is why you get an error when you try to use them as two different identifiers.
• @ on its own is not a valid identifier, which is why the compiler is very confused if you try to declare a method with just it as its name and throws up a lot of parsing errors. (That behavior could be improved, but that's just a matter of getting nicer error messages, not a bug in the compiler.)

If you want to know more, all this is explained in the Identifiers section of the spec.