C# in Depth

310 CHAPTER 11 Query expressions and LINQ to Objects{}Console.WriteLine(entry.Key.Name);foreach (var summary in entry){Console.WriteLine(" {0}", summary);}Console.WriteLine();I’ve highlighted the differences between listing 11.18 and listing 11.17 in bold. Havingprojected a defect to just its summary, the embedded sequence in each entry is just anIEnumerable. In this case, the compiler uses an overload of GroupBy withanother parameter to represent the projection. The query expression in listing 11.18is translated into the following expression:SampleData.AllDefects.Where(defect => defect.AssignedTo != null).GroupBy(defect => defect.AssignedTo,defect => defect.Summary)There are more complex overloads of GroupBy available as extension methods onIEnumerable, but they aren’t used by the C# 3 compiler when translating queryexpressions. You can call them manually, of course—if you find you want more powerfulgrouping behavior than query expressions provide natively, then they’re worthlooking into.Grouping clauses are relatively simple but very useful. Even in our defect-trackingsystem, you could easily imagine wanting to group defects by project, creator, severity,or status, as well as the assignee we’ve used for these examples.So far, we’ve ended each query expression with a select or group … by clause, andthat’s been the end of the expression. There are times, however, when you want to domore with the results—and that’s where query continuations are used.11.6.2 Query continuationsQuery continuations provide a way of using the result of one query expression as theinitial sequence of another. They apply to both group … by and select clauses, andthe syntax is the same for both—you simply use the contextual keyword into and thenprovide the name of a new range variable. That range variable can then be used in thenext part of the query expression.The C# 3 specification explains this in terms of a translation from one queryexpression to another, changingintofirst-query into identifiersecond-query-bodyfrom identifier in (first-query)second-query-bodyAn example will make this a lot clearer. Let’s go back to our grouping of defects byassignee, but this time imagine we only want the count of the defects assigned to eachLicensed to Rhona Hadida