Sunday, March 02, 2008

C# 4 idea: Iterator blocks and parameter checking

by skeet via Jon Skeet's Coding Blog : C# on 3/2/2008 7:54:44 PM

Iterator blocks have an interesting property: they defer execution. When the method (or property) is called, none of your code is executed - it only starts running when MoveNext() is first called.

Deferred execution is a great thing in many ways, but it's a pain when it comes to parameter checking. If you check parameters within an iterator block, you've effectively left a timebomb - the error will be potentially reported a long way from the original source of the problem. (Side-note: this is why I prefer using a cast to as when I know what type something really should be - I'd rather get an InvalidCastException immediately than a NullReferenceException later on.)

One solution to this is to check the parameters in the public method and then call a private method which is implemented with an iterator block. That's a bit ugly though. It would be nice to be able to specify the parameter checking in an initial block which could only occur at the start of the method - something like this simple implementation of Repeat:

I've chosen yield do for two reasons: it fits in with the rest of the yield pattern, and by using an existing keyword I suspect it reduces/eliminates the chance of this being a breaking change. It's not ideal from the point of self-description, but do is the closest keyword I could find when I looked at the spec. In a language with begin as a keyword, that would probably be a better choice - or initial. fixed appeals in that the code is fixed in the method rather than being shunted off into the compiler-generated type, but the normal use of fixed is far too different to make this an appealing choice. Any other suggestions are very welcome :)

Is this an important enough change to make it worth including in C# 4? I'm not sure. The negative side is that I suspect relatively few people use iterator blocks much, so it may not have a very wide benefit. The positive side is that it only actually makes any difference to those who do use iterator blocks, and I believe almost all of them would find it a welcome addition. It's simple to understand, and it makes iterator blocks much easier to use in a "correct" manner when any form of initial checking is involved.

Note, mostly to myself and Marc Gravell: I haven't checked, but I suspect most of Push LINQ is broken in this respect.

Further note to self - I really need to fix my code formatter to treat yield as a contextual keyword.