C# in Depth

284 CHAPTER 11 Query expressions and LINQ to ObjectsListing 11.1Trivial query to print the list of usersvar query = from user in SampleData.AllUsersselect user;foreach (var user in query){Console.WriteLine(user);}The query expression is the part highlighted in bold. I’ve overridden ToString foreach of the entities in the model, so the results of listing 11.1 are as follows:User: Tim Trotter (Tester)User: Tara Tutu (Tester)User: Deborah Denton (Developer)User: Darren Dahlia (Developer)User: Mary Malcop (Manager)User: Colin Carton (Customer)You may be wondering how useful this is as an example: after all, we could have justused SampleData.AllUsers directly in our foreach statement. However, we’ll use thisquery expression—however trivial—to introduce two new concepts. First we’ll look atthe general nature of the translation process the compiler uses when it encounters aquery expression, and then we’ll discuss range variables.11.2.2 Compiler translations as the basis of query expressionsThe C# 3 query expression support is based on the compiler translating query expressionsinto “normal” C# code. It does this in a mechanical manner that doesn’t try tounderstand the code, apply type inference, check the validity of method calls, or anyof the normal business of a compiler. That’s all done later, after the translation. Inmany ways, this first phase can be regarded as a preprocessor step. The compiler translateslisting 11.1 into listing 11.2. before doing the real compilation.Listing 11.2The query expression of listing 11.1 translated into a method callvar query = SampleData.AllUsers.Select(user => user);foreach (var user in query){Console.WriteLine(user);}The C# 3 compiler translates the query expression into exactly that code before properlycompiling it further. In particular, it doesn’t assume that it should use Enumerable.Select, or that List will contain a method called Select. It merely translates thecode and then lets the next phase of compilation deal with finding an appropriatemethod—whether as a straightforward member or as an extension method. The parametercan be a suitable delegate type or an Expression for an appropriate type T.This is where it’s important that lambda expressions can be converted intoboth delegate instances and expression trees. All the examples in this chapter willLicensed to Rhona Hadida