C# in Depth

304 CHAPTER 11 Query expressions and LINQ to Objectsthe first two from clauses, then cross join the resulting sequence with the next fromclause, and so on. Each extra from clause adds its own range variable.Listing 11.15 shows a simple (but useless) cross join in action, producing asequence where each entry consists of a user and a project. I’ve deliberately pickedtwo completely unrelated initial sequences to show that no matching is performed.Listing 11.15Cross joining users against projectsvar query = from user in SampleData.AllUsersfrom project in SampleData.AllProjectsselect new { User=user, Project=project };foreach (var pair in query){Console.WriteLine("{0}/{1}",pair.User.Name,pair.Project.Name);}The output of listing 11.15 begins like this:Tim Trotter/Skeety Media PlayerTim Trotter/Skeety TalkTim Trotter/Skeety OfficeTara Tutu/Skeety Media PlayerTara Tutu/Skeety TalkTara Tutu/Skeety OfficeFigure 11.8 shows the sequences involved to get this result.If you’re familiar with SQL, you’re probably quite comfortable so far—it looks justlike a Cartesian product obtained from a query specifying multiple tables. Indeed,most of the time that’s exactly how cross joins are used. However, there’s more poweravailable when you want it: the right sequence used at any particular point in time candepend on the “current” value of the left sequence. When this is the case, it’s not across join in the normal sense of the term. The query expression translation is thesame whether or not we’re using a true cross join, so we need to understand the morecomplicated scenario in order to understand the translation process.Before we dive into the details, let’s see the effect it produces. Listing 11.16 shows asimple example, using sequences of integers.Listing 11.16Cross join where the right sequence depends on the left elementvar query = from left in Enumerable.Range(1, 4)from right in Enumerable.Range(11, left)select new { Left=left, Right=right };foreach (var pair in query){Console.WriteLine("Left={0}; Right={1}",pair.Left, pair.Right);}Licensed to Rhona Hadida