C# in Depth

294 CHAPTER 11 Query expressions and LINQ to ObjectsSo, that’s what the query expression looks like—but what does the compiler do? It simplycalls the OrderBy and ThenBy methods (or OrderByDescending/ThenBy-Descending for descending orders). Our query expression is translated intoSampleData.AllDefects.Where (bug => bug.Status != Status.Closed).Where (bug => bug.AssignedTo == tim).OrderByDescending (bug => bug.Severity).ThenBy (bug => bug.LastModified)Now that we’ve seen an example, let’s look at the general syntax of orderby clauses.They’re basically the contextual keyword orderby followed by one or more orderings.An ordering is just an expression (which can use range variables) optionally followed bydescending, which has the obvious meaning. The translation for the first ordering is acall to OrderBy or OrderByDescending, and any other orderings are translated using acall to ThenBy or ThenByDescending, as shown in our example.The difference between OrderBy and ThenBy is quite simple: OrderBy assumes ithas primary control over the ordering, whereas ThenBy understands that it’s subservientto one or more previous orderings. For LINQ to Objects, ThenBy is only defined asan extension method for IOrderedEnumerable, which is the type returned byOrderBy (and by ThenBy itself, to allow further chaining).It’s very important to note that although you can use multiple orderbyclauses, each one will start with its own OrderBy or OrderByDescendingclause, which means the last one will effectively “win.” There may be somereason for including multiple orderby clauses, but it would be veryunusual. You should almost always use a single clause containing multipleorderings instead.As noted in chapter 10, applying an ordering requires all the data tobe loaded (at least for LINQ to Objects)—you can’t order an infinitesequence, for example. Hopefully the reason for this is obvious—youdon’t know whether you’ll see something that should come at the start of the resultingsequence until you’ve seen all the elements, for example.We’re about halfway through learning about query expressions, and you may be surprisedthat we haven’t seen any joins yet. Obviously they’re important in LINQ just asthey’re important in SQL, but they’re also complicated. I promise we’ll get to them indue course, but in order to introduce just one new concept at a time, we’ll detour vialet clauses first. That way we can learn about transparent identifiers before we hit joins.Warning:only use oneorderbyclause!11.4 Let clauses and transparent identifiersMost of the rest of the operators we still need to look at involve transparent identifiers.Just like range variables, you can get along perfectly well without understanding transparentidentifiers, if you only want to have a fairly shallow grasp of query expressions.If you’ve bought this book, I hope you want to know C# 3 at a deeper level, which will(among other things) enable you to look compilation errors in the face and knowwhat they’re talking about.Licensed to Rhona Hadida