Idempotent modifiers in C#

Question

I noticed that if I write something like:

static void Main(string[] args)
{
 const const const bool flag = true;
}

The compiler doesn't warn me of the multiple consts. So this seems to mimic C modifiers, as they are idempotent.

However, if I write:

private readonly readonly int a;

The compiler does warn me of the duplicated readonly.

So what's going on here? Are modifiers idempotent or not?

---

^{csc version 1.0.0.50618}

• Jon Skeet: That the first line compiles is a bug in whichever compiler you're using - which you haven't told us. It doesn't compile with Roslyn...
• Jakub Lortz: @JonSkeet It doesn't compile if you try to put it as field. It does compile inside a method static void Main(string[] args) { const const const bool flag = true; }
• waka: @JakubLortz: Nope, still creates an error (Type expected) in VS2010.
• Jon Skeet: @JakubLortz: Ah, thanks for that. This is why short but complete examples are important. Will double check that it's invalid according to the spec, but it does look like it's just a compiler bug.
• DavidG: @SteffenWinkler No, it applies only to C# version 6, it's nothing to do with the version of Visual Studio. I could have written that code with Notepad/Sublime and compiled it that way.

Answer 1

It's a bug in the compiler - at least in Roslyn version 1.0.0.50618. From section 8.5.2 of the C# 5 specification:

A local-constant-declaration declares one or more local constants.

local-constant-declaration:
const type constant-declarators

constant-declarators:
constant-declarator
constant-declarators , constant-declarator

constant-declarator: identifier = constant-expression

As you can see, that grammar doesn't allow for const const const bool flag = true;.

I've filed a bug against Roslyn so that it can get fixed.

• DavidG: This pull request looks to be the fix.