C# in Depth

288 CHAPTER 11 Query expressions and LINQ to Objectsbecomes easier to understand a query expression that has a slightly more complicatedprojection. Listing 11.4 prints out just the names of our users.Listing 11.4Query selecting just the names of the usersvar query = from user in SampleData.AllUsersselect user.Name;foreach (string name in query){Console.WriteLine(name);}This time we’re using user.Name as the projection, and we can see that the result is asequence of strings, not of User objects. The translation of the query expression followsthe same rules as before, and becomesSampleData.AllUsers.Select(user => user.Name)The compiler allows this, because the Select extension method as applied to AllUserseffectively has this signature, acting as if it were a member of IEnumerable:IEnumerable Select (Func selector)NOTEExtension methods pretending to be instance methods—Just to be clear, Selectisn’t a member of IEnumerable itself, and the real signature has anIEnumerable parameter at the start. However, the translation processdoesn’t care whether the result is a call to a normal instance method oran extension method. I find it easier to pretend it’s a normal method justfor the sake of working out the query translation. I’ve used the same conventionfor the remainder of the chapter.The type inference described in chapter 9 kicks in, converting the lambda expressioninto a Func by deciding that the user parameter must be of type User,and then inferring that the return type (and thus TResult) should be string. This iswhy lambda expressions allow implicitly typed parameters, and why there are suchcomplicated type inference rules: these are the gears and pistons of the LINQ engine.NOTE Why do you need to know all this? You can almost ignore what’s going onwith range variables for a lot of the time. You may well have seen many,many queries and understood what they achieve without ever knowingabout what’s going on behind the scenes. That’s fine for when things areworking (as they tend to with examples in tutorials), but it’s when thingsgo wrong that it pays to know about the details. If you have a queryexpression that won’t compile because the compiler is complaining thatit doesn’t know about a particular identifier, you should look at the rangevariables involved.So far we’ve only seen implicitly typed range variables. What happens when weinclude a type in the declaration? The answer lies in the Cast and OfType standardquery operators.Licensed to Rhona Hadida