Why doesn't the compiler give any errors or warnings when using this hack?

问题

In my other question, I found a hack to make this syntax work in MonoDevelop editor:

// hack to make MonoDevelop recognize nameof syntax from C#6.0
using nameof = System.Func<string>;

The C# compilers (Mono and VS) don't give any warnings or errors, and usages of the nameof keyword also work normally. My question is why.

回答1:

I'm not a language lawyer but I believe the reason your code works is that nameof is a contextual keyword

Let's take a step back to a more general case. If you try to create a using alias directive for the keyword if you get an error...

using if = System.Func<string>;  // "CS1001: Identifier expected" error

… unless you prefix the name with @ ...

using @if = System.Func<string>;  // No "CS1001: Identifier expected" error

Similarly you get a CS1003 error if you try to declare a variable of the aliased type ...

if foo = () => "Hello, World";  // "CS1003: Syntax error, '(' expected" error

… unless you prefix the name with an @ sign …

@if foo = () => "Hello, World";  // No "CS1003: Syntax error, '(' expected" error

Contextual keywords on the other hand do not need to be prefixed by @ …

using nameof = System.Func<string>;

nameof bar = () => "Hello, World!";

Console.WriteLine(nameof(nameof));

来源：https://stackoverflow.com/questions/51457554/why-doesnt-the-compiler-give-any-errors-or-warnings-when-using-this-hack