C# in Depth

298 CHAPTER 11 Query expressions and LINQ to Objectsto a sequence of cities by saying that the birth date of the person is the same as thepopulation of the city—it doesn’t make any sense.The syntax for an inner join looks more complicated than it is:[query selecting the left sequence]join right-range-variable in right-sequenceon left-key-selector equals right-key-selectorSeeing equals as a contextual keyword rather than using symbols can be slightly disconcerting,but it makes it easier to distinguish the left key selector from the right keyselector. Often (but not always) at least one of the key selectors is a trivial one that justselects the exact element from that sequence.Let’s look at an example from our defect system. Suppose we had just added thenotification feature, and wanted to send the first batch of emails for all the existingdefects. We need to join the list of notifications against the list of defects, where theirprojects match. Listing 11.12 performs just such a join.Listing 11.12Joining the defects and notification subscriptions based on projectvar query = from defect in SampleData.AllDefectsjoin subscription in SampleData.AllSubscriptionson defect.Project equals subscription.Projectselect new { defect.Summary, subscription.EmailAddress };foreach (var entry in query){Console.WriteLine("{0}: {1}", entry.EmailAddress, entry.Summary);}Listing 11.12 will show each of the media player bugs twice—once for “mediabugs@skeetysoft.com”and once for “theboss@skeetysoft.com” (because the boss reallycares about the media player project).In this particular case we could easily have made the join the other way round,reversing the left and right sequences. The result would have been the same entriesbut in a different order. The implementation in LINQ to Objects returns entries sothat all the pairs using the first element of the left sequence are returned (in theorder of the right sequence), then all the pairs using the second element of the leftsequence, and so on. The right sequence is buffered, but the left sequence isstreamed—so if you want to join a massive sequence to a tiny one, it’s worth using thetiny one as the right sequence if you can.One error that might trip you up is putting the key selectors the wrong way round.In the left key selector, only the left sequence range variable is in scope; in the rightkey selector only the right range variable is in scope. If you reverse the left and rightsequences, you have to reverse the left and right key selectors too. Fortunately thecompiler knows that it’s a common mistake and suggests the appropriate course ofaction.Just to make it more obvious what’s going on, figure 11.6 shows the sequences asthey’re processed.Licensed to Rhona Hadida