Why is nameof(object) not allowed?

Question

In C# 6.0 you can write this:

var instance = default(object);
var type = typeof(object);

They have the same result of:

var instance = default(System.Object);
var type = typeof(System.Object);

But you can't write this:

var name = nameof(object);

It generates the following error:

Invalid expression term 'object'.

But you can still write this:

var name = nameof(System.Object);

Why nameof(object) does not compile?

Dominy

Try: nameof(Object) instead. nameof() doesn't work on synonyms, only on the original classnames. – Nickey

@CodeCaster, to get the class Object's name as string – Nickey

@CodeCaster Jeroen is right. Try it if you don't believe it. – Testify

@CodeCaster also on classnames. I use it for logging purposes. Whenever I rename a class, this will be changed to.. Same applies to nameof(int) vs nameof(Int32) – Nickey

Not sure why this was closed. It's a perfectly valid question, if not a bit short. If anything, it goes against the common idea that Object and object are entirely interchangeable. See here. This question shows that in fact, they are not the same. – Silicious

@Jeroen alright, TIL. – Tilden

@Silicious agree there. The difference is that object is a synonym for Object and nameof() doesn't work on synonyms. – Nickey

object is a keyword in the C# language. Keywords are rather a big deal to compilers, they disambiguate syntax and help generate good error messages. But with the hang-up that keywords are not identifiers so nameof() can't work. – Suprematism

@HansPassant how can keywords "help generate good error messages?" – Dominy

Keywords are the rock-solid foundations upon which a language is built. The parts of a statement that have a completely unambiguous meaning and the parser can rely on to mean only one thing. If a parser can't rely on such guarantees then it has to guess what the programmer intended. Guesses never produce good error messages, it might guess wrong and send the programmer into a rabbit hole. It would technically not be impossible for the compiler to still recognize object in the specific case of parsing the nameof operand. It would however greatly uglify the expression parser. – Suprematism

@Hans you mean like ideone (Mono)'s "CS1525: Unexpected symbol ), expecting ." after nameof(object)? :) – Tilden

Yes, good example :) Mono treats it like an identifier instead of a keyword, tsk, tsk. – Suprematism

"It would however greatly uglify the expression parser" @HansPassant are you saying that adding the recognition of keywords in the nameof() statement would have slowed down the parsing of the code? I'm not expert of low-level stuffs like compiler parsing, so it's a honest question. – Dominy

Speed is not an issue. The expression parser would have to accept keywords like object, string, int, etc. But only in the specific case of it being used to parse the nameof operand. And still deal with the wonky cases like object int and produce a decent error message for them. The expression parser is one of the most difficult parts of a compiler, there is lots of expressive power in a language like C#. The less special cases, the lower the risk that it accidentally accepts malformed code or spits out a bad diagnostic. – Suprematism

Answer 1

The difference is that object is a synonym for the class Object and nameof() doesn't work on synonyms.

Same applies to nameof(int) vs nameof(Int32)

Nickey

Relevant documentation: msdn.microsoft.com/en-au/library/dn986596.aspx "The following are worth mentioning that produce errors: predefined types (for example, int or void)" – Silicious

@Silicious well that's the reference, not the specification. Technically, System.Object is a predefined simple type, and object its alias, so neither should work according to that description... – Tilden