MSDN Magazine - July 2008 - (Page 38)

erty of the category, allowing it to work on each of the products within the category. The sample returns a sequence containing the following list of strings. The call to SelectMany flattened the original category/product hierarchy into a single list of values: 0. 0. 0. 0. 0. 0. 1. 1. 1. 1. 1. Category Category Category Category Category Category Category Category Category Category Category 6: 6: 6: 6: 6: 6: 7: 7: 7: 7: 7: Mishi Kobe Niku Alice Mutton Thüringer Rostbratwurst Perth Pasties Tourtière Pâté chinois Uncle Bob's Organic Dried Pears Tofu Rössle Sauerkraut Manjimup Dried Apples Longlife Tofu The Enumerable class also provides similar methods that return a single element if possible and a default member of the sequence’s type if not. These methods are Enumerable.FirstOrDefault, Enu merable.LastOrDefault, Enumerable.SingleOrDefault, and Enu merable.ElementAtOrDefault. Figure 2 demonstrates the behavior of these methods. Running the sample places the following data into the output stream: ALFKI: Maria Anders ALFKI: Maria Anders Last in USA: Karl Jablonski Sequence contains no elements Specified argument was out of the range of valid values. Parameter name: index CustomerID = XXXXX is Nothing: True Customer 2 in USA is Nothing: False Customer 200 in USA is Nothing: True Retrieving a Single Element Often, you will need to retrieve and work with a single element from a sequence. The Enumerable class provides several methods that allow you to filter the contents of a sequence to a single ele ment. The Enumerable.First and Enumerable.Last methods re turn the first or last elements in the sequence, respectively. The Enumerable.Single method returns an element given a function or lambda expression that specifies a single element. The Enumer able.ElementAt method returns a single element at a particular in dex within the sequence. Each of these methods throws an exception if the requested el ement doesn’t exist in the sequence—or, for the Single method, if the restriction results in either zero elements or multiple elements. Each of these methods also allows you to specify a lambda expres sion or function to restrict the input data. In any case, the resulting sequence must contain a single element. Figure 2 Retrieving Elements with Enumerable Methods ' From SingleElementDemo in the sample: Dim oneCustomer As Customer Dim sw As New StringWriter Dim db As New SimpleDataContext Dim query0 = From cust In db.Customers Dim query1 = _ From cust In db.Customers _ Where cust.CustomerID = "ALFKI" Dim query2 = _ From cust In db.Customers _ Where cust.Country = "USA" Dim query3 = _ From cust In db.Customers _ Where cust.CustomerID = "XXXXX" ' This works fine, finds the customer whose CustomerID is ALFKI: oneCustomer = query1.Single sw.WriteLine("ALFKI: {0}", oneCustomer.ContactName) ' You can supply an expression to filter, when calling ' the First, Last, Single, and so on, methods: oneCustomer = query0.Single( _ Function(cust) cust.CustomerID = "ALFKI") sw.WriteLine("ALFKI: {0}", oneCustomer.ContactName) ' This works fine, finds the last customer in the USA: oneCustomer = query2.Last Filtering Sequences Use the Enumerable.Where, Enumerable.Distinct, or Enumer able.OfType method to filter the contents of an existing sequence, returning a subset of the original data in an output sequence. The OfType method filters the input sequence based on a specified type. Imagine that you want to perform an operation on only a particu lar type of control on a form. Using the OfType method, you can restrict the collection of controls exposed by the form and iterate through only the relevant subset of controls. The following example fills a generic List with data and retrieves an output list of integers, and then retrieves a list of strings: ' From OfTypeDemo in the sample: Dim items As New List(Of Object) items.Add("January") items.Add(0) sw.WriteLine("Last in USA: {0}", oneCustomer.ContactName) ' These two raise exceptions: Try ' There's no customer use customer ID is "XXXXX": oneCustomer = query3.Single Catch ex As Exception sw.WriteLine(ex.Message) End Try Try ' There aren't 200 customers in the USA: oneCustomer = query2.ElementAt(200) Catch ex As Exception sw.WriteLine(ex.Message) End Try ' These don't raise exceptions, but return default values ' if they fail: oneCustomer = query3.SingleOrDefault sw.WriteLine("CustomerID = XXXXX is Nothing: {0}", _ oneCustomer Is Nothing) ' Because there are at least two customers in the USA, ' method returns a valid, single customer: oneCustomer = query2.ElementAtOrDefault(2) sw.WriteLine("Customer 2 in USA is Nothing: {0}", _ oneCustomer Is Nothing) oneCustomer = query2.ElementAtOrDefault(200) sw.WriteLine("Customer 200 in USA is Nothing: {0}", _ oneCustomer Is Nothing) 38 msdn magazine 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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