MSDN Magazine - September 2008 - (Page 41)

Figure 12 Group by Category ' From ComplexSetOperationDemo in the sample: Dim groupedProducts = _ (From prod In db.Products _ Where prod.UnitsInStock < 10).GroupBy( _ Function(prod) prod.CategoryID, _ Function(prod) _ New With {prod.ProductName, prod.UnitsInStock}, _ Function(catID, group) New With _ {.CategoryID = catID, .Count = group.Count(), _ .Items = group}) sw = New StringWriter For Each grouping In groupedProducts sw.WriteLine("Category {0}: {1} item(s)", _ grouping.CategoryID, grouping.Count) For Each item In grouping.Items sw.WriteLine("{0}{1} ({2})", vbTab, _ item.ProductName, item.UnitsInStock) Next Next Figure 13 Joining Category and Product Sequences ' From ComplexSetOperation in the sample: categories = db.Categories.Where(Function(cat) cat.CategoryID < 5) products = db.Products Dim groupedResults = _ categories.GroupJoin(products, _ Function(cat) cat.CategoryID, _ Function(prod) prod.CategoryID, _ Function(cat, group) _ New With {.Category = cat, .Count = group.Count, _ .Items = group}) sw = New StringWriter For Each grouping In groupedResults sw.WriteLine("{0}: {1} item(s)", _ grouping.Category.CategoryName, grouping.Count) For Each item In grouping.Items sw.WriteLine("{0}{1} ({2})", vbTab, _ item.ProductName, item.UnitsInStock) Next Next Results Category 2: 3 item(s) Chef Anton’s Gumbo Mix (0) Northwoods Cranberry Sauce (6) Louisiana Hot Spiced Okra (4) Category 3: 2 item(s) Sir Rodney’s Scones (3) Scottish Longbreads (6) Category 4: 2 item(s) Gorgonzola Telino (0) Mascarpone Fabioli (9) Category 6: 3 item(s) Alice Mutton (0) Thüringer Rostbratwurst (0) Perth Pasties (0) Category 7: 1 item(s) Longlife Tofu (4) Category 8: 1 item(s) Rogede sild (5) Results Beverages: 12 item(s) Chai (39) Chang (17) ‘ Items removed here. . . Lakkalikööri (57) Condiments: 12 item(s) Aniseed Syrup (13) Chef Anton’s Cajun Seasoning (53) ‘ Items removed here. . . Original Frankfurter grüne Soße (32) Confections: 13 item(s) Pavlova (29) ‘ Items removed here. . . Scottish Longbreads (6) Dairy Products: 10 item(s) Queso Cabrales (22) ‘ Items removed here. . . Mozzarella di Giovanni (14) Running the sample code in Figure 11 produces the output at the bottom of the figure. The Enumerable.GroupBy method groups the input sequence by a key value, creating groupings of items. Each group in the output sequence contains a “header” item and an Items property that provides access to the grouped items.To call the Enumerable.GroupBy method, you must supply at least three functions. The first provides the grouping key, the second provides the resulting item in the group, and the third provides the header contents. The code in Figure 12 groups a sequence of products by the category ID and displays the results. Running the code creates the output at the bottom of the figure, grouped by category ID. Finally, the Enumerable.GroupJoin method combines the functionality of the GroupBy and Join methods. This method correlates two sequences based on matching keys and groups the results in the right-hand sequence. Nothing quite like this method exists in standard database functionality, and you may find this method useful when working with related sets of data. To call the GroupJoin method, you must supply three functions. The first function defines the key on the parent side. The second function defines the key on the child side. The third function projects the data, given the parent row and the child group, into the output format for the “one” set of rows. The two keys must be of the same type, and if the type isn’t a simple type, you must provide a custom comparer so that the keys can be compared. msdnmagazine.com The sample code in Figure 13 joins category and product sequences using the category ID as the correlating key. The code specifies that the output rows contain a Category property (which contains the entire category row), a Count property (which contains the number of child rows), and an Items property (which exposes the group of child rows). The sample iterates through each grouping, prints out information about the header row, and then displays information about each child row. After running the sample, the StringWriter contains the results shown at the bottom of Figure 13. Summing It Up Through its clever use of extension methods, the Enumerable class makes working with many different types of collections, lists, and sequences simpler. In addition, the Enumerable class provides the “heart” of working with LINQ queries. If you’re building applications that use data from a database or any type of data structure in Visual Studio® 2008, it’s worth completely internalizing the capabilities of the rich Enumerable class. You’ll create more efficient code, and you’ll create it faster. I constantly search for ways to make my code more declarative, and the Enumerable class makes it far easier to avoid looping and allows you to write less code.  Ken Getz is a Senior Consultant with MCW Technologies and a courseware author for AppDev (www.appdev.com). He is coauthor of ASP.NET Developers Jumpstart, Access Developer’s Handbook, and VBA Developer’s Handbook, 2nd Edition. Reach him at keng@mcwtech.com. September 2008 41 http://www.appdev.com http://msdnmagazine.com

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

MSDN Magazine - September 2008 Contents Toolbox CLR Inside Out Data Points Advanced Basics Office Space Cutting Edge Hierarchy ID New Features for Microsoft SQL Server 2008 Prism Data Services Advanced WPF Test Run Security Briefs Foundations { End Bracket }

MSDN Magazine - September 2008

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