MSDN Magazine - July 2008 - (Page 36)

From prod In db.Products _ Where prod.CategoryID = 1 _ Order By prod.ProductName _ Select prod.ProductName If you’ve thought much about this, you’ve probably realized that, behind the scenes, the Visual Basic compiler must convert the SQL like syntax into some set of method calls on some instance of a type. Indeed, in this example, the Products property of the Simple DataContext class is of the List(Of Product) type and, as you might have guessed, the List(Of T) type implements the IEnumerable(Of T) interface. The compiler converts the LINQ keywords into cor responding calls to extension methods on the Enumerable type. Specifically, you could create the same exact set of instructions by rewriting the previous LINQ query like this: Dim query = _ db.Products _ .Where(Function(prod) CBool(prod.CategoryID = 1)) _ .OrderBy(Function(prod) prod.ProductName) _ .Select(Function(prod) prod.ProductName) only to create LINQ queries but also to manipulate the behavior of arrays and other data structures. If you examine the documentation for the extension methods on the Enumerable class, you’ll find that each member is Shared. As a result, you have two options for calling a method. First, you can call it as a method of the declaring type, like this, passing in the instance of the extended class (assuming that the results variable is of a type can be processed by the Count method): Dim itemCount = _ Enumerable.Count(results, _ Function(prod) prod.CategoryID = 3) You can also call Count as if it were an instance method on the col lection itself. When you call the extension method this way, you don’t need to supply the first parameter: Dim itemCount = _ results.Count(Function(prod) _ prod.CategoryID = 3) If you examine the intermediate language (IL) created by the com piler, using the LINQ query that employs Visual Basic keywords, you will see that the compiler converts the Visual Basic keywords into calls to the corresponding methods of the Enumerable class. Doing this results in the extension of the System.Collections.Ge neric.List(Of T) class behavior. (As a matter of fact, it’s easy to de termine this—look in the documentation for this particular class’s members, and you’ll see a list of extension methods, most of which are defined by the Enumerable class.) What does this all mean to you? Because the Enumerable class’s extension methods can process many other classes—including Ar ray and List—you can use methods of the Enumerable class not Figure 1 LINQ Keyword Links to Enumerable Methods Visual Basic LINQ Query Keyword Aggregate … Into … All Aggregate … Into … Any Aggregate … Into … Average Aggregate … Into … Count Aggregate … Into … LongCount Aggregate … Into … Max Aggregate … Into … Min Aggregate … Into … Sum Distinct Group By Group Join Order By Order By … Descending Order By (with multiple fields) Order By … Descending (with multiple fields) Select Skip Take Take While Where 36 msdn magazine Enumerable Method All Any Average Count LongCount Max Min Sum Distinct GroupBy GroupJoin OrderBy OrderByDescending ThenBy ThenByDescending Select Skip Take TakeWhile Where Visual Basic and C# handle the conversion from keywords into extension method calls differently. Visual Basic provides more key words that you can use within your LINQ queries than does C#. For example, you can create a LINQ query in Visual Basic using the Take keyword, like this: Dim db As New SimpleDataContext() Dim query = _ From prod In db.Products _ Where prod.CategoryID = 1 _ Order By prod.ProductName _ Take 10 _ Select prod.ProductName You could also use the Take extension method, provided by the Enumerable class, like this: Dim query = _ (From prod In db.Products _ Where prod.CategoryID = 1 _ Order By prod.ProductName _ Select prod.ProductName).Take(10) Because C# doesn’t supply the Take keyword, you would have no other choice than to call the Take method explicitly. Figure 1 lists the mappings between Visual Basic LINQ query keywords and the corresponding methods of the Enumerable class. Creating Sequences The Enumerable class provides several Shared methods that are not extension methods but exist for you to create new sequences. I will start by examining these simple methods. The Enumerable.Range method creates a new sequential list of integers. You specify the starting value and the number of items in the list. The method returns an IEnumerable(Of Integer) sequence. You can use this method to solve the original problem in this ar ticle (that is, retrieving a list of numbers between two endpoints, in random order). The following code fills the list using the Range method, then calls the Enumerable.OrderBy method with a simple lambda ex pression to arrange the sequence in random order: ' From RangeDemo in the sample: Dim rnd As New System.Random Dim items = Enumerable.Range(1, 10) Dim randomList = _ items.OrderBy(Function() rnd.Next()) Running the sample procedure multiple times returns different Advanced Basics

Table of Contents Feed for the Digital Edition of MSDN Magazine - July 2008

MSDN Magazine - July 2008 Contents Toolbox CLR Inside Out Flex Your Data Data Points Advanced Basics Office Space Cutting Edge Data Services ADO.NET Data and WPF Transactions WCF P2P Test Run Security Briefs Foundations .NET Matters {End Bracket}

MSDN Magazine - July 2008

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