MSDN Magazine - September 2008 - (Page 35)

KEN GETz AdvAnced BAsics The LINQ Enumerable Class, Part 2 In the last installment, I took you on a quick tour through approximately half of the methods in the System.Linq.Enumerable class. This class, which provides all the extension methods that make LINQ queries in Visual Basic® and C#, also extends other classes, such as List(Of T) and Array, making it possible to use querying semantics with objects that wouldn’t otherwise support querying methods. You can review that column in the July 2008 issue at msdn.microsoft.com/magazine/cc700332. Figure 1 Creating a Dictionary ' From ToDictionaryDemo in the sample: Dim db As New SimpleDataContext Dim someProducts = _ From prod In db.Products _ Where prod.CategoryID = 1 Dim productsDictionary = _ someProducts.ToDictionary( _ Function(prod) prod.ProductID) ' Display the contents of the dictionary: Dim sw As New StringWriter For Each kvp In productsDictionary sw.WriteLine("{0}: {1}", _ kvp.Key, kvp.Value.ProductName) Next This time, I’ll complete the tour of the methods, looking at methods that handle converting, positioning, calculating, and performing set operations on sequences of objects. (You should download the sample application, which contains all the code I show here. Following along in the app will help you understand how the code works, especially since you can alter it to experiment as you go.) Figure 2 Product Comparer Public Class ProductComparer Implements IEqualityComparer(Of Product) Public Function Equals1( _ ByVal x As Product, ByVal y As Product) As Boolean _ Implements IEqualityComparer(Of Product).Equals Return x.ProductID.Equals(y.ProductID) End Function Public Function GetHashCode1( _ ByVal obj As Product) As Integer _ Implements IEqualityComparer(Of Product).GetHashCode Return obj.GetHashCode End Function End Class Converting Sequences If you need to take the results of working with an enumerable sequence and pass them to some method that requires a specific type, or you need to call a method of a specific type using data that’s stored in an enumerable sequence, you may need to call one of the Enumerable class methods that convert the data to a different type. For example, imagine that you have customer data in a sequence, and you’d like a comma-delimited list of customer names. Although you can solve this problem several other ways (as you’ll see when you investigate the Aggregate method later in the column), the String.Join method solves this problem handily. The problem is that this method only accepts an array of strings as input. The answer is the Enumerable.ToArray method, which converts the collection to an array: 'From ToArrayDemo in the sample: Dim db As New SimpleDataContext Dim customers = _ From cust In db.Customers _ Where cust.Country = "France" _ Select cust.ContactName Dim nameList As String = String.Join(", ", customers.ToArray()) Using the data in the Northwind sample database, this code fills the nameList variable with the following text: Frédérique Citeaux, Laurence Lebihan, Janine Labrune, Martine Rancé, Carine Schmitt, Daniel Tonini, Annette Roulet, Marie Bertrand, Dominique Perrier, Mary Saveley, Paul Henriot Enumerable.ToDictionary method provides several overloads, allowing you to specify various combinations of key selector, value selector, key comparer, and value comparer methods. Imagine that you’ve extracted product information from the Northwind sample database and you’d like an in-memory dictionary using product ID values as the key. The code in Figure 1 handles this task for you. This code takes the contents of the someProducts variable and converts it to a Dictionary, using the ProductID field as the key value. The code then loops through all the items in the dictionary, printing the key (that is, the ProductID field) and one field from the value of each dictionary item. Send your questions and comments for Ken to basics@microsoft.com. Code download available at msdn.microsoft.com/magazine/cc135911. September 2008 35 You can convert from an enumerable sequence into a generic Dictionary, although you must at least supply a function that indicates how you want to generate the key values. Note that the http://msdn.microsoft.com/magazine/cc700332 http://msdn.microsoft.com/magazine/cc135911

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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