C# in Depth

Groupings and continuations307In just five lines of code we have retrieved, parsed, and filtered a whole collection oflog files, returning a sequence of entries representing errors. Crucially, we haven’t hadto load even a single full log file into memory all in one go, let alone all of the files—all the data is streamed.Having tackled joins, the last items we need to look at are slightly easier to understand.We’re going to look at grouping elements by a key, and continuing a queryexpression after a group … by or select clause.11.6 Groupings and continuationsOne common requirement is to group a sequence of elements by one of its properties.LINQ makes this easy with the group … by clause. As well as describing this finaltype of clause, we’ll also revisit our earliest one (select) to see a feature called querycontinuations that can be applied to both groupings and projections. Let’s start with asimple grouping.11.6.1 Grouping with the group … by clauseGrouping is largely intuitive, and LINQ makes it simple. To group a sequence in aquery expression, all you need to do is use the group … by clause, with this syntax:group projection by groupingThis clause comes at the end of a query expression in the same way a select clausedoes. The similarities between these clauses don’t end there: the projection expressionis the same kind of projection a select clause uses. The outcome is somewhat different,however.The grouping expression determines what the sequence is grouped by—the key ofthe grouping. The overall result is a sequence where each element is itself a sequenceof projected elements, and also has a Key property, which is the key for that group;this combination is encapsulated in the IGrouping interface, whichextends IEnumerable.Let’s have a look at a simple example from the SkeetySoft defect system: groupingdefects by their current assignee. Listing 11.17 does this with the simplest form of projection,so that the resulting sequence has the assignee as the key, and a sequence ofdefects embedded in each entry.Listing 11.17Grouping defects by assignee—trivial projectionvar query = from defect in SampleData.AllDefectswhere defect.AssignedTo != nullgroup defect by defect.AssignedTo;foreach (var entry in query){Console.WriteLine(entry.Key.Name);foreach (var defect in entry){Console.WriteLine(" ({0}) {1}",Uses key of eachentry: the assigneeEDBFilters outunassigned defectsGroups byassigneeCIterates over entry’ssubsequenceLicensed to Rhona Hadida