C# in Depth

308 CHAPTER 11 Query expressions and LINQ to Objects}}Console.WriteLine();defect.Severity,defect.Summary);Listing 11.17 might be useful in a daily build report, to quickly see what defects eachperson needs to look at. We’ve filtered out all the defects that don’t need any moreattention B and then grouped using the AssignedTo property. Although this timewe’re just using a property, the grouping expression can be anything you like—it’s justapplied to each entry in the incoming sequence, and the sequence is grouped basedon the result of the expression. Note that grouping cannot stream the results,although it streams the input, applying the key selection and projection to each elementand buffering the grouped sequences of projected elements.The projection we’ve applied in the grouping C is trivial—it just selects the originalelement. As we go through the resulting sequence, each entry has a Key property,which is of type User D, and each entry also implements IEnumerable,which is the sequence of defects assigned to that user E.The results of listing 11.17 start like this:Darren Dahlia(Showstopper) MP3 files crash system(Major) Can't play files more than 200 bytes long(Major) DivX is choppy on Pentium 100(Trivial) User interface should be more caramellyAfter all of Darren’s defects have been printed out, we see Tara’s, then Tim’s, and soon. The implementation effectively keeps a list of the assignees it’s seen so far, andadds a new one every time it needs to. Figure 11.9 shows the sequences generatedthroughout the query expression, which may make this ordering clearer.Within each entry’s subsequence, the order of the defects is the same as the orderof the original defect sequence. If you actively care about the ordering, considerexplicitly stating it in the query expression, to make it more readable.If you run listing 11.17, you’ll see that Mary Malcop doesn’t appear in the output atall, because she doesn’t have any defects assigned to her. If you wanted to produce afull list of users and defects assigned to each of them, you’d need to use a group joinlike the one used in listing 11.14.The compiler always uses a method called GroupBy for grouping clauses. When theprojection in a grouping clause is trivial—in other words, when each entry in the originalsequence maps directly to the exact same object in a subsequence—the compileruses a simple method call, which just needs the grouping expression, so it knows howto map each element to a key. For instance, the query expression in listing 11.17 istranslated into this nonquery expression:SampleData.AllDefects.Where(defect => defect.AssignedTo != null).GroupBy(defect => defect.AssignedTo)Licensed to Rhona Hadida