One of the nice things about a project as large as the Roslyn project is that you have an opportunity to really think hard about your past mistakes and hopefully fix them. When I was working on getting these error messages reported in Roslyn I realized that trying to match exactly the behavior of the original compiler would be actively making the world a worse place, so I took a big step back and started sending a lot of emails to Mads, Neal, Anthony and the rest of the Roslyn gang to try and get a better design worked out. All the godawful nonsense I told you about in the previous two episodes will be fixed in Roslyn.

Continue reading →

Last time I gave you the challenge to find a case where the same simple name means two different things, without introducing a new local/parameter/range variable into scope, that produces an error. It seems like it ought to be impossible; if nothing new has been introduced to a local scope then how can name resolution choose two different things? The relevant section of the C# specification (7.6.2.1 Invariant meaning in blocks) only gives the example I gave last time, of a local having the same name as a field.

The key to solving the riddle is a little-known rule about resolving a name from a set of possible class members: Continue reading →

There’s a saying amongst programming language designers that every language is a response to previous languages; the designers of C# were, and still are, very deliberate about learning from the mistakes and successes of similar languages such as C, C++, Java, Scala and so on. One feature of C# that I have a love-hate relationship with is a direct response to a dangerous feature of C++, whereby the same name can be used to mean two different things throughout a block. I’ve already discussed the relevant rules of C# at length, so review my earlier posting before you read on.

OK, welcome back. Summing up:

• C++ allows one name to mean two things when one local variable shadows another.
• C++ allows one name to mean two things when one usage of a name refers to a member and a local variable of the same name is declared later.
• Both of these features make it harder to understand, debug and maintain programs.
• C# makes all that illegal; every simple name must have a unique meaning throughout its containing block, which implies that the name of a local variable may not shadow any other local or be used to refer to any member.

I have a love-hate relationship with this “unique meaning” feature, which we are going to look at in absurd depth in this series.
Continue reading →