C# in Depth

350 CHAPTER 12 LINQ beyond collectionsAsParallel call, we would only use a single thread, but AsParallel and the resultingcalls to the ParallelEnumerable extension methods means that the work is split intoup to three threads. 12One caveat about this: unless you specify that you want the results in the sameorder as the strings in the original sequence, PLINQ will assume you don’t mind gettingresults as soon as they’re available, even if results from earlier elements haven’tbeen returned yet. You can prevent this by passing QueryOptions.PreserveOrderingas a parameter to AsParallel.There are other subtleties to using PLINQ, such as handling the possibility of multipleexceptions occurring instead of the whole process stopping on the first problematicelement—consult the documentation for further details when ParallelExtensions is fully released. More examples of PLINQ are included in the downloadablesource code.As you can see, PLINQ isn’t a “data source”—it’s a kind of meta-provider, alteringhow a query is executed. Many developers will never need it—but I’m sure that thosewho do will be eternally grateful for the coordination it performs for them behindthe scenes.These won’t be the only new providers Microsoft comes up with—we should expectnew APIs to be built with LINQ in mind, and that should include your own code as well.I confidently expect to see some weird and wonderful uses of LINQ in the future.12.6 SummaryPhew! This chapter has been the exact opposite of most of the rest of the book.Instead of focusing on a single topic in great detail, we’ve covered a vast array of LINQproviders, but at a shallow level.I wouldn’t expect you to feel particularly familiar with any one of the specific technologieswe’ve looked at here, but I hope you’ve got a deeper understanding of whyLINQ is important. It’s not about XML, or in-memory queries, or even SQL queries—it’sabout consistency of expression, and giving the C# compiler the opportunity to validateyour queries to at least some extent, regardless of their final execution platform.You should now appreciate why expression trees are so important that they areamong the few framework elements that the C# compiler has direct intimate knowledgeof (along with strings, IDisposable, IEnumerable, and Nullable, for example).They are passports for behavior, allowing it to cross the border of the local machine,expressing logic in whatever foreign tongue is catered for by a LINQ provider.It’s not just expression trees—we’ve also relied on the query expression translationemployed by the compiler, and the way that lambda expressions can be converted toboth delegates and expression trees. Extension methods are also important, as withoutthem each provider would have to give implementations of all the relevant methods. If12 I’ve explicitly specified the number of threads in this example to force parallelism even on a single-core system.If the number of threads isn’t specified, the system acts as it sees fit, depending on the number of coresavailable and how much other work they have.Licensed to Rhona Hadida