C# in Depth

290 CHAPTER 11 Query expressions and LINQ to Objectsforeach (string start in strings){Console.WriteLine(start);}The output of listing 11.6 is “Fir,” “Sec,” “Thi”—but what’s more interesting is thetranslated query expression, which islist.Cast().Select(entry => entry.Substring(0,3));Without the cast, we wouldn’t be able to call Select at all, because the extensionmethod is only defined for IEnumerable rather than IEnumerable. Even whenyou’re using a strongly typed collection, you might still want to use an explicitly typedrange variable, though. For instance, you could have a collection that is defined to bea List but you know that all the elements are instances ofMyImplementation. Using a range variable with an explicit type of MyImplementationallows you to access all the members of MyImplementation without manually insertingcasts all over the code.We’ve covered a lot of important conceptual ground so far, even though wehaven’t achieved any impressive results. To recap the most important points briefly:■ LINQ is based on sequences of data, which are streamed wherever possible.■ Creating a query doesn’t immediately execute it: most operations use deferredexecution.■ Query expressions in C# 3 involve a preprocessing phase that converts theexpression into normal C#, which is then compiled properly with all the normalrules of type inference, overloading, lambda expressions, and so forth.■ The variables declared in query expressions don’t act like anything else: theyare range variables, which allow you to refer to data consistently within thequery expression.I know that there’s a lot of somewhat abstract information to take in. Don’t worry ifyou’re beginning to wonder if LINQ is worth all this trouble. I promise you that it is.With a lot of the groundwork out of the way, we can start doing genuinely usefulthings—like filtering our data, and then ordering it.11.3 Filtering and ordering a sequenceYou may be surprised to learn that these two operations are some of the simplest toexplain in terms of compiler translations. The reason is that they always return asequence of the same type as their input, which means we don’t need to worry aboutany new range variables being introduced. It also helps that we’ve seen the correspondingextension methods in chapter 10.11.3.1 Filtering using a where clauseIt’s remarkably easy to understand the where clause. The format is justwhere filter-expressionLicensed to Rhona Hadida