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How LINQ extends .NET
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This section gives a refresher on the features we introduced in chapter 2 and puts them into the big picture so you can get a clear idea of how they all work together when used with LINQ. We ll also enumerate the elements LINQ brings to the party, which we ll detail in the rest of this chapter.
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Refresher on the language extensions
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As a refresher, let s sum up the significant additions to the languages that you discovered in chapter 2:
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Implicitly typed local variables Object initializers Lambda expressions Extension methods Anonymous types
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These additions are what we call language extensions, the set of new language features and syntactic constructs added to C# and VB.NET to support LINQ. All of these extensions require new versions of the C# and VB.NET compilers, but no new IL instructions or changes of the .NET runtime.
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LINQ building blocks
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These language extensions are full-fledged features that can be used in code that has nothing to do with LINQ. They are however required for LINQ to work, and you ll use them a lot when writing language-integrated queries. In order to introduce LINQ concepts and understand why they are important, we ll dissect a code sample throughout this chapter. We ll keep the same subject as in chapter 2: filtering and sorting a list of running processes. Here is the code sample we ll use:
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static void DisplayProcesses() { var processes = Process.GetProcesses() .Where(process => process.WorkingSet64 > 20*1024*1024) .OrderByDescending(process => process.WorkingSet64) .Select(process => new { process.Id, Name=process.ProcessName }); ObjectDumper.Write(processes); }
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The portion of code in bold is a LINQ query. If you take a close look at it, you can see all the language enhancements we introduced in the previous chapter, as shown in figure 3.1. In the figure, you should clearly see how everything dovetails to form a complete solution. You can now understand why we called the language enhancements key components for LINQ.
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The language extensions all in one picture
Introducing sequences
The key elements of the LINQ foundation
More features and concepts are required for LINQ to work than those we ve just listed. Several concepts specifically related to queries are also required:
We ll start by explaining what sequences are and how they are used in LINQ queries. You ll also encounter query expressions. This is the name for the from where select syntax you ve already seen. We ll explore query operators, which represent the basic operations you can perform in a LINQ query. We ll also explain what deferred query execution means, and why it is important. In order to enable deferred query execution, LINQ uses expression trees. We ll see what expression trees are and how LINQ uses them.
You need to understand these features in order to be able to read and write LINQ code, as we ll do in the next chapters.
Introducing sequences
The first LINQ concept we ll present in this chapter is the sequence. In order to introduce sequences and understand why they are important, let s dissect listing 3.1.
Listing 3.1 Querying a list of processes using extension methods
var processes = Process.GetProcesses() .Where(process => process.WorkingSet64 > 20*1024*1024) .OrderByDescending(process => process.WorkingSet64) .Select(process => new { process.Id, Name=process.ProcessName });
C D E
B C D E
Get a list of running processes Filter the list Sort the list Keep only the IDs and names
LINQ building blocks
To precisely understand what happens under the covers, let s analyze this code step by step, in the order the processing happens. We ll start by looking at IEnumerable<T>, a key interface you ll find everywhere when working with LINQ. We ll also provide a small refresher on iterators and then stress how iterators allow deferred query execution.
IEnumerable<T>
The first thing you need to understand in listing 3.1 is what the call to Process.GetProcesses B returns and how it is used. The GetProcesses method of the System.Diagnostics.Process class returns an array of Process objects. This is not surprising and probably wouldn t be interesting, except that arrays implement the generic IEnumerable<T> interface. This interface, which appeared with .NET 2.0, is key to LINQ. In our particular case, an array of Process objects implements IEnumerable<Process>. The IEnumerable<T> interface is important because Where C, OrderByDescending D, Select E, and other standard query operators used in LINQ queries expect an object of this type as a parameter. Listing 3.2 shows how the Where method is defined, for instance.
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