C# 'dynamic' keyword... is it really a RESERVED keyword or just a identifier that means something special when used as type?

Question

I have a C# 4.0 parser. It accepts 'dynamic' as a keyword as a type. My parser trips over statements found in working C# 3.0 programs of the form of:

dynamic = <exp> ;

So, it dynamic really a keyword? Or can it still be used as an arbitrary identifier name? (If so, why isn't 'int' treated the same way)?

Is there a reference spec somewhere that states whether dynamic is keyword? The latest ECMA C# 4 specification doesn't even mention 'dynamic', and the best I can find at the MS site is a "preliminary specification" which says its a keyword but I suspect that's just sloppy writing.

Answer 1

dynamic is a contextual keyword as of C# 4.0. The fourth edition of the ECMA C# spec does not refer to C# 4.0. (Note the 2006 publication date.)

MSDN describes it as a keyword.

It's also in this table of contextual keywords. (Scroll down.)

Answer 2

It's not a "normal" keyword like if, for etc.

It's a contextual keyword like yield, from etc.

In particular, this compiles:

object dynamic = 0; // dynamic is a contextual keyword

but this doesn't:

object string = 0; // string is a regular keyword

You'd have to "escape" string like this:

object @string = 0;

This makes a lot of sense in terms of backward compatibility: it's unlikely that many people will have created a type called dynamic (which would cause ambiguity; IIRC the "real" type wins in this case) whereas it's very likely that existing code uses variables called dynamic.

In some ways it doesn't even need to be a contextual keyword (and I don't believe the specification ever explicitly refers to it as such) - you can think of it as the name of a type which is always available (like string and object). I would imagine that string etc were made keywords from v1 to avoid confusion, but adding genuine keywords (which can't be used as identifiers) would have a high compatibility cost now.