C# in Depth

332 CHAPTER 12 LINQ beyond collections12.2.4 The fake query provider in actionListing 12.8 shows a simple query expression, which (supposedly) finds all the stringsin our fake source beginning with “abc” and projects the results into a sequence of thelengths of the matching strings. We iterate through the results, but don’t do anythingwith them, as we know already that they’ll be empty. Of course, we have no sourcedata, and we haven’t written any code to do any real filtering—we’re just loggingwhich calls are made by LINQ in the course of creating the query expression and iteratingthrough the results.Listing 12.8A simple query expression using the fake query classesvar query = from x in new FakeQuery()where x.StartsWith("abc")select x.Length;foreach (int i in query) { }What would you expect the results of running listing 12.8 to be? In particular, whatwould you like to be logged last, at the point where we’d normally expect to do somereal work with the expression tree? Here are the results of listing 12.8, reformattedslightly for clarity:FakeQueryProvider.CreateQueryExpression=FakeQuery.Where(x => x.StartsWith("abc"))FakeQueryProvider.CreateQueryExpression=FakeQuery.Where(x => x.StartsWith("abc")).Select(x => x.Length)FakeQuery.GetEnumeratorExpression=FakeQuery.Where(x => x.StartsWith("abc")).Select(x => x.Length)The two important things to note are that GetEnumerator is only called at the end, noton any intermediate queries, and that by the time GetEnumerator is called we have allthe information present in the original query expression. We haven’t manually had tokeep track of earlier parts of the expression in each step—a single expression tree capturesall the information “so far” at any point in time.Don’t be fooled by the concise output, by the way—the actual expression tree isquite deep and complicated, particularly due to the where clause including an extramethod call. This expression tree is what LINQ to SQL would be examining to workout what query to execute. LINQ providers could build up their own queries (in whateverform they may need) as calls to CreateQuery are made, but usually looking at thefinal tree when GetEnumerator is called is simpler, as all the necessary information isavailable in one place.The final call logged by listing 12.8 was to FakeQuery.GetEnumerator, and you maybe wondering why we also need an Execute method on IQueryProvider. Well, not allquery expressions generate sequences—if you use an aggregation operator such as Sum,Licensed to Rhona Hadida