C# in Depth

LINQ beyond .NET 3.5349All of the LINQ providers we’ve seen so far have acted on a particular data source,and performed the appropriate transformations. Our next topic is slightly different—but it’s one I’m particularly excited about.PARALLEL LINQ (PLINQ)Ten years ago, the idea of even fairly low-to-middling laptops having dual-processorcores would have seemed ridiculous. Today, that’s taken for granted—and if the chipmanufacturers’ plans are anything to go by, that’s only the start. Of course, it’s only usefulto have more than one processor core if you’ve got tasks you can run in parallel.Parallel LINQ, or PLINQ for short, is a project with one “simple” goal: to execute LINQto Objects queries in parallel, realizing the benefits of multithreading with as few headachesas possible. At the time of this writing, PLINQ is targeted to be released as part ofParallel Extensions, the next generation of .NET concurrency support. The sample Idescribe is based on the December 2007 Community Technology Preview (CTP).Using PLINQ is simple, if (and only if) you have to perform the same task on eachelement in a sequence, and those tasks are independent. If you need the result ofone calculation step in order to find the next, PLINQ is not for you—but many CPUintensivetasks can in fact be done in parallel. To tell the compiler to use PLINQ, youjust need to call AsParallel (an extension method on IEnumerable) on your datasource, and let PLINQ handle the threading. As with IQueryable, the magic is justnormal compiler method resolution: AsParallel returns an IParallelEnumerable,and the ParallelEnumerable class provides static methods to handle the standardquery operators.Listing 12.22 demonstrates PLINQ in an entirely artificial way, putting threads tosleep for random periods instead of actually hitting the processor hard.Listing 12.22Executing a LINQ query on multiple threads with Parallel LINQstatic int ObtainLengthSlowly(string name){Thread.Sleep(StaticRandom.Next(10000));return name.Length;}...string[] names = {"Jon", "Holly", "Tom", "Robin", "William"};var query = from name in names.AsParallel(3)select ObtainLengthSlowly(name);foreach (int length in query){Console.WriteLine(length);}Listing 12.22 will print out the length of each name. We’re using a random 11 sleep tosimulate doing some real work within the call to ObtainLengthSlowly. Without the11 The StaticRandom class used for this is merely a thread-safe wrapper of static methods around a normalRandom class. It’s part of my miscellaneous utility library.Licensed to Rhona Hadida