C# in Depth

286 CHAPTER 11 Query expressions and LINQ to Objectsstatic void Main(){var source = new Dummy();Creates sourceto be queried}}var query = from dummy in sourcewhere dummy.ToString()=="Ignored"select "Anything";Running listing 11.3 prints “Where called” and then “Select called” just as we’dexpect, because the query expression has been translated into this code:var query = source.Where(dummy => dummy.ToString()=="Ignored").Select(dummy => "Anything");Calls methods via aquery expressionOf course, we’re not doing any querying or transformation here, but it shows how thecompiler is translating our query expression. If you’re puzzled as to why we’ve selected"Anything" instead of just dummy, it’s because a projection of just dummy (which is a “donothing” projection) would be removed by the compiler in this particular case. We’lllook at that later in section 11.3.2, but for the moment the important idea is the overalltype of translation involved. We only need to learn what translations the C# compilerwill use, and then we can take any query expression, convert it into the form thatdoesn’t use query expressions, and then look at what it’s doing from that point of view.Notice how we don’t implement IEnumerable at all in Dummy. The translationfrom query expressions to “normal” code doesn’t depend on it, but in practice almostall LINQ providers will expose data either as IEnumerable or IQueryable (whichwe’ll look at in chapter 12). The fact that the translation doesn’t depend on any particulartypes but merely on the method names and parameters is a sort of compile-timeform of duck typing. This is similar to the same way that the collection initializers presentedin chapter 8 find a public method called Add using normal overload resolutionrather than using an interface containing an Add method with a particular signature.Query expressions take this idea one step further—the translation occurs early in thecompilation process in order to allow the compiler to pick either instance methods orextension methods. You could even consider the translation to be the work of a separatepreprocessing engine.NOTE Why from … where … select instead of select … from … where? Manydevelopers find the order of the clauses in query expressions confusingto start with. It looks just like SQL—except back to front. If you look backto the translation into methods, you’ll see the main reason behind it. Thequery expression is processed in the same order that it’s written: we startwith a source in the from clause, then filter it in the where clause, thenproject it in the select clause. Another way of looking at it is to considerthe diagrams throughout this chapter. The data flows from top to bottom,and the boxes appear in the diagram in the same order as their correspondingclauses appear in the query expression. Once you get overany initial discomfort due to unfamiliarity, you may well find thisapproach appealing—I certainly do.Licensed to Rhona Hadida