C# in Depth

Simple beginnings: selecting elements283As you can see, we’re not recording an awful lot of data. In particular, there’s no realhistory to the defects, but there’s enough here to let us demonstrate the queryexpression features of C# 3. For the purposes of this chapter, all the data is stored inmemory. We have a class named SampleData with properties AllDefects, AllUsers,AllProjects, and AllSubscriptions, which each return an appropriate type ofIEnumerable. The Start and End properties return DateTime instances for thestart and end of August respectively, and there are nested classes Users andProjects within SampleData to provide easy access to a particular user or project.The one type that may not be immediately obvious is NotificationSubscription:the idea behind this is to send an email to the specified address every time a defectis created or changed in the relevant project.There are 41 defects in the sample data, created using C# 3 object initializers. All ofthe code is available on the book’s website, but I won’t include the sample data itselfin this chapter.Now that the preliminaries are dealt with, let’s get cracking with some queries!11.2 Simple beginnings: selecting elementsHaving brought up some general LINQ concepts beforehand, I’ll introduce the conceptsthat are specific to C# 3 as they arise in the course of the rest of the chapter. We’regoing to start with a simple query (even simpler than the ones we’ve seen so far in thechapter) and work up to some quite complicated ones, not only building up your expertiseof what the C# 3 compiler is doing, but also teaching you how to read LINQ code.All of our examples will follow the pattern of defining a query, and then printingthe results to the console. We’re not interested in binding queries to data grids or anythinglike that—it’s all important, but not directly relevant to learning C# 3.We can use a simple expression that just prints out all our users as the startingpoint for examining what the compiler is doing behind the scenes and learning aboutrange variables.11.2.1 Starting with a source and ending with a selectionEvery query expression in C# 3 starts off in the same way—stating the source of asequence of data:from element in sourceThe element part is just an identifier, with an optional type name before it. Most ofthe time you won’t need the type name, and we won’t have one for our first example.Lots of different things can happen after that first clause, but sooner or later youalways end with a select clause or a group clause. We’ll start off with a select clauseto keep things nice and simple. The syntax for a select clause is also easy:select expressionThe select clause is known as a projection. Combining the two together and using themost trivial expression we can think of gives a simple (and practically useless) query,as shown in listing 11.1.Licensed to Rhona Hadida