C# in Depth

Filtering and ordering a sequence291The compiler translates this into a call to the Where method with a lambda expression,which uses the appropriate range variable as the parameter and the filterexpression as the body. The filter expression is applied as a predicate to each elementof the incoming stream of data, and only those that return true are present inthe resulting sequence. Using multiple where clauses results in multiple chainedWhere calls—only elements that match all of the predicates are part of the resultingsequence. Listing 11.7 demonstrates a query expression that finds all open defectsassigned to Tim.Listing 11.7Query expression using multiple where clausesUser tim = SampleData.Users.TesterTim;var query = from bug in SampleData.AllDefectswhere bug.Status != Status.Closedwhere bug.AssignedTo == timselect bug.Summary;foreach (var summary in query){Console.WriteLine(summary);}The query expression in listing 11.7 is translated intoSampleData.AllDefects.Where (bug => bug.Status != Status.Closed).Where (bug => bug.AssignedTo == tim).Select(bug => bug.Summary)The output of listing 11.7 is as follows:Installation is slowSubtitles only work in WelshPlay button points the wrong wayWebcam makes me look baldNetwork is saturated when playing WAV fileOf course, we could write a single where clause that combined the two conditions asan alternative to using multiple where clauses. In some cases this may improve performance,but it’s worth bearing the readability of the query expression in mind, too.Once more, this is likely to be a fairly subjective matter. My personal inclination is tocombine conditions that are logically related but keep others separate. In this case,both parts of the expression deal directly with a defect (as that’s all our sequence contains),so it would be reasonable to combine them. As before, it’s worth trying bothforms to see which is clearer.In a moment, we’ll start trying to apply some ordering rules to our query, but firstwe should look at a small detail to do with the select clause.11.3.2 Degenerate query expressionsWhile we’ve got a fairly simple translation to work with, let’s revisit a point I glossedover earlier in section 11.2.2 when I first introduced the compiler translations. So far,Licensed to Rhona Hadida