Fugato

Here’s one techie posting to alienate my regular readers (both of them) … I’ve encountered an apparent bug in the C# compiler in Visual Studio .Net 2003. Witness:

public interface ProductRow
{
	int Count { get ; }
	int Price { get ; }
}
public class ProductRowCollection
{
	public ProductRow this[int index]
	{
		get { return null; }
	}
	public int TotalPrice()
	{
		return ( this[0].Count * this[0].Price );
	}
}

(Yeah, I know the code is silly; I stripped it down for display purposes.)

This causes the compiler to bork with the following errors:

test.cs(14,20): error CS1513: } expected
test.cs(14,33): error CS1031: Type expected
test.cs(14,42): error CS1519: Invalid token ‘)’ in class, struct, or interface member declaration
test.cs(16,1): error CS1022: Type or namespace definition, or end-of-file expected

The first error is signalled on the reference to the Count property getter, a perfectly decent member of ProductRow. I am fairly sure the code is valid. The error seems to indicate something going awry in lexing.

It goes away if I put in a + instead of the * … maybe the compiler is confusing itself by mistaking the multiplication operator for a pointer indirection operator, for some reason? The C# language specification seems a little vague on the precedence and associativity of the pointer indirection operator; is it possible that this really is invalid code and the compiler is correctly rejecting it but just giving a shabby error message?

It also goes away if I comment it out and back in! This is really odd: I get this in Visual Studio, indicated with a red line under the Count reference, and I try to build and I get the compiler error … and then I comment the line out, then uncomment it again, and I don’t get a red line and it compiles! It makes sense that the syntax highlighting “snaps out of it” when I comment and uncomment … but the compiler too?

Of course that’s a shoddy, brittle, voodoo workaround, and sure enough, when I check it in it gets picked up by the automated build system and the same compiler error comes up there.

A sane workaround is to extract ProductRow row = this[0]; and write row.Count * row.Price … this way no error is raised. Probably judiciously applied parentheses would achieve the same.

I googled a little bit, but didn’t find anybody talking about this kind of glitch. So I hereby ceremoniously christen it The Marteinsson Mess, after my coworker who stumbled on it.

Update: this has been fixed in .Net Framework SDK v2.0 Beta 2.

This entry was posted on Friday, September 2nd, 2005 at 11:01 pm and is filed under Microsoft, Tech. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.