MSDN Magazine - July 2008 - (Page 37)

results each time, but you should see a list like the following each time you execute the code: 6, 8, 7, 1, 2, 9, 10, 4, 3, 5 10. 11. 12. 13. Liz Nixon Liu Wong Helvetius Nagy Karl Jablonski The Enumerable.Reverse method returns its input sequence in reverse order. The following code calls the Reverse method both as a Shared method of the Enumerable class and as an instance method on the list resulting from calling the Enumerable.Range method: 'From ReverseDemo in the sample: Dim items = Enumerable.Range(1, 10) Dim reversed = items.Reverse() reversed = Enumerable.Reverse(items) However you call the Reverse method, you’ll end up with the following list: 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 The Enumerable.SelectMany method takes a collection of items, each of which can be a collection of items, and collapses the twodimensional input into a single-dimensional output sequence. This method can be useful if you need to take, for example, a list of categories, each of which contains products, and end up with a list of products with corresponding category information. The following example retrieves a list of categories for which the category contains less than seven products, and projects the result into a single list containing information about each product, along with category information: ' From SelectManyDemo in the sample: Dim db As New SimpleDataContext Dim categories = _ From cat In db.Categories Where cat.Products.Count < 7 Dim manyResults As IEnumerable(Of String) = _ categories.SelectMany(Function(cat, index) _ cat.Products.Select( _ Function(prod As Product) String.Format("{0}. Category {1}: {2}", _ index, prod.CategoryID, prod.ProductName))) The Enumerable.Repeat method creates a list containing the value you supply, repeated as many times as you specify: 'From RepeatDemo in the sample: Dim repeated = Enumerable.Repeat("Hello", 5) After calling the previous code, the repeated variable contains the following items: Hello, Hello, Hello, Hello, Hello Use the Enumerable.Empty method to create an empty IEnumerable(Of T) that’s set up to contain the specified data type. Using this method, you could create the empty instance of the collection and add items to it as necessary. By initializing the collection to an empty collection, you never have to worry about whether the reference is Nothing: 'From EmptyDemo in the sample: Dim emptyList = _ Enumerable.Empty(Of Customer)() In this case, the lambda expression passed to the SelectMany method accepts a reference to a specific category, working through each category in turn. This particular overload of the SelectMany method passes the index of the category along with the category to the lambda expression. Given the category and its index, the lambda expression calls the Select method on the Products prop- Selecting Sequences The Enumerable class provides two different extension methods that allow you to select a sequence of elements, projecting the elements from the input sequence into an output sequence. The Enumerable.Select method simply projects each element into a new form. The following example takes a sequence containing customers in the U.S. from the Northwind Customers table, then projects the results into a sequence that contains a numbered list of contact names: ' From SelectDemo in the sample: Dim db As New SimpleDataContext Dim results = _ From cust In db.Customers _ Where cust.Country = "USA" Dim selectResults = results.Select( _ Function(cust, index) _ String.Format("{0}. {1}", _ index + 1, cust.ContactName)) Note that the Select method has multiple overloaded versions. After running this code, selectResults contains a list of values resembling the following: 1. 2. 3. 4. 5. 6. 7. 8. 9. Howard Snyder Yoshi Latimer John Steel Jaime Yorres Fran Wilson Rene Phillips Paula Wilson Jose Pavarotti Art Braunschweiger msdnmagazine.com July 2008 37 http://www.ektron.com/msdn http://www.ektron.com/msdn http://msdnmagazine.com

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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