MSDN Magazine - July 2008 - (Page 40)

' From DistinctDemo in the sample: Dim db As New SimpleDataContext Dim countries As IEnumerable(Of String) = _ db.Customers _ .Select(Function(cust) cust.Country) _ .Distinct() countries = customers _ .Distinct(New CustomerCountryComparer) _ .Select(Function(cust) cust.Country) a sequence of customers and then applies ordering using several of these methods: ' From OrderByDemo in the sample: Dim db As New SimpleDataContext Dim customers = _ From cust In db.Customers _ Where cust.ContactTitle = "Owner" ' Sort first by country, then by city descending, then by name: Dim results = _ customers _ .OrderBy(Function(cust) cust.Country) _ .ThenByDescending(Function(cust As Customer) cust.City) _ .ThenBy(Function(cust As Customer) cust.ContactName) _ .Select(Function(cust) String.Format("({0}, {1}) {2}", _ cust.Country, cust.City, cust.ContactName)) You may need to handle empty sequences gracefully, and the Enumerable.DefaultIfEmpty method returns either the entire in put sequence or a defaultvalued instance if the sequence is empty. Using this method, you’re guaranteed that you always have at least a single item of the sequence’s type. The procedure in Figure 4 demonstrates one use for the Default IfEmpty method. This code fills the noCustomers variable with an empty list (there are no customers whose country is “XXX”), then creates the noCustomers1 list calling the DefaultIfEmpty method. Running the sample code produces the following output: noCustomers contains 0 element(s). noCustomers1 contains 1 element(s). XXXXX noCustomers1 contains one element (a Customer with no informa tion filled in). You can call the DefaultIfEmpty method supplying a default value for the singleton item, as in the second method call. Note that Visual Basic won’t complain if you don’t specify the data types for the parameter to the lambda expression in the ThenBy and ThenByDescending method calls, but if you don’t, you won’t get any IntelliSense® to help you. Only the first ordering method can infer the data type of the lambda expression parameter. Odd ly enough, the Select method correctly infers the type, even at the end of the long chained list of methods. The sample sorts the cus tomers whose title is “Owner” first by country, then descending by city, and, finally, by contact name within each city: (Denmark, Kobenhavn) Jytte Petersen (France, Paris) Marie Bertrand (France, Nantes) Janine Labrune (France, Marseille) Laurence Lebihan (Germany, Köln) Henriette Pfalzheim (Mexico, México D.F.) Ana Trujillo (Mexico, México D.F.) Antonio Moreno (Mexico, México D.F.) Miguel Angel Paolino (Norway, Stavern) Jonas Bergulfsen (Poland, Warszawa) Zbyszek Piestrzeniewicz Ordering Sequences You can use Enumerable.OrderBy, Enumerable.ThenBy, Enumer able.OrderByDescending, and Enumerable.ThenByDescending to supply an initial ordering and then one or more secondary order ings (in ascending or descending order). This sample starts with Figure 5 Finding a Sequence ' From AnyDemo in sample: Dim db As New SimpleDataContext Dim results = _ From product In db.Products _ Where product.CategoryID = 1 ' Determine if a list has any elements: Dim anyElements = results.Any() ' Call as extension method or shared method ' of IEnumerable: Dim matchingElements = _ results.Any(Function(prod) prod.ProductName.StartsWith("M")) matchingElements = _ Enumerable.Any(results, _ Function(prod As Product) prod.ProductName.StartsWith("M")) Verifying Sequences Figure 6 Finding Items in a Sequence ' From ContainsDemo in the sample: ' Simple check to see if list contains a value: Dim numbers = Enumerable.Range(1, 10) Dim contains5 = numbers.Contains(5) ' More complex objects require more complex comparison: Dim db As New SimpleDataContext Dim results = _ From product In db.Products _ Where product.CategoryID = 1 Dim item As New Product _ With {.ProductID = 1, .ProductName = "Chai"} Dim containsChai = results.Contains(item, New ProductComparer()) Some applications require you to verify that specific conditions are met within a sequence. Do any of the elements meet a criterion? Do all the elements meet a criterion? Does a particular item appear within the sequence? Are two sequences equal? The Enumerable class provides methods that provide all this information. To determine whether a sequence contains any elements at all, call the Enumerable.Any method without any parameters. To sup ply a condition and to determine whether any elements in the se quence match the condition, pass a function to the method. The method returns True if any elements of the input sequence exist or match the supplied condition (see Figure 5). To determine whether all members of a sequence meet a criterion, call the Enumerable.All method, supplying a function that specifies the condition. This example determines if all the elements in the results collection have a UnitsInStock value greater than 5: ' From AllDemo in sample: ' Is it true that all products have more than 5 units in stock? Dim db As New SimpleDataContext Dim results = _ From product In db.Products _ Where product.CategoryID = 1 Dim allElements = _ results.All(Function(prod) CBool(prod.UnitsInStock > 5)) To determine whether a sequence contains a particular element, call Enumerable.Contains. If you’re searching for a simple value, like a string or an integer, you can use the default comparer, and Advanced Basics 40 msdn magazine

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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.