C# in Depth

Translations using IQueryable and IQueryProvider333Count, or Average, we’re no longer really creating a “source”—we’re evaluating a resultimmediately. That’s when Execute is called, as shown by listing 12.9 and its output.Listing 12.9 IQueryProvider.Executevar query = from x in new FakeQuery()where x.StartsWith("abc")select x.Length;double mean = query.Average();// OutputFakeQueryProvider.CreateQueryExpression=FakeQuery.Where(x => x.StartsWith("abc"))FakeQueryProvider.CreateQueryExpression=FakeQuery.Where(x => x.StartsWith("abc")).Select(x => x.Length)FakeQueryProvider.ExecuteExpression=FakeQuery.Where(x => x.StartsWith("abc")).Select(x => x.Length).Average()The FakeQueryProvider can be quite useful when it comes to understanding what theC# compiler is doing behind the scenes with query expressions. It will show the transparentidentifiers introduced within a query expression, along with the translated callsto SelectMany, GroupJoin, and the like.12.2.5 Wrapping up IQueryableWe haven’t written any of the significant code that a real query provider would needin order to get useful work done, but hopefully our fake provider has given youinsight into how LINQ providers are given the information from query expressions.It’s all built up by the Queryable extension methods, given an appropriate implementationof IQueryable and IQueryProvider.We’ve gone into a bit more detail in this section than we will for the rest of thechapter, as it’s involved the foundations that underpin the LINQ to SQL code we sawearlier. You’re unlikely to want to write your own query provider—it takes a lot of workto produce a really good one—but this section has been important in terms of conceptualunderstanding. The steps involved in taking a C# query expression and (atexecution time) running some SQL on a database are quite profound and lie at theheart of the big features of C# 3. Understanding why C# has gained these features willhelp keep you more in tune with the language.In fact, LINQ to SQL is the only provider built into the framework that actually usesIQueryable—the other “providers” are just APIs that play nicely with LINQ to Objects.I don’t wish to diminish their importance—they’re still useful. However, it does meanthat you can relax a bit now—the hardest part of the chapter is behind you. We’re stillstaying with database-related access for our next section, though, which looks at LINQto DataSet.Licensed to Rhona Hadida