MSDN Magazine - August 2008 - (Page 56)

tive values and associations described in the service. This allows developers to navigate through a graph of associated entities using the object model directly. .NET Client Library The ADO.NET Data Services .NET client library presents a programming model that is familiar to developers who write apps using the .NET Framework and data services. Under the covers, the client library uses HTTP and the AtomPub format, so it works naturally over both corporate networks and Internet environments, requiring only simple HTTP-level connectivity to the data service, whether direct or indirect (for example, through proxies). client library discussed earlier, the AJAX client library abstracts the details of HTTP so that the application developer can work directly with JavaScript objects rather than manually parsing and creating HTTP requests and responses. A number of Web pages are also available on the CodePlex site that explain how to use the data service library for AJAX applications (see codeplex.com). In Figure 5, the queries against the service were built by calling the DataServiceQuery.CreateQuery method and passing in a URI query. As an alternative, the library also allows you to formulate data service queries using LINQ. The client library maps the LINQ statement to a URI in the target data service and retrieves the specified resources as .NET objects. The following code shows how to retrieve all the customers in the city of London with the results ordered by company name: var q = from c in ctx.Customers where c.City == "London" orderby c.CompanyName select c; foreach (var cust in q) { Console.WriteLine(cust.CompanyName); } Querying a Data Service Any query that requires two or more pivots cannot currently be mapped to a URI. The client library can be used from any project type, including Windows Forms, Windows Presentation Foundation (WPF), and Web projects. In order to use the client library above, you will need to add a reference to the System.Data.Services.Client.dll assembly. Silverlight Client Library The Silverlight Client doesn’t ship as a part of ADO.NET Data Services, but rather ships as part of the Silverlight 2 SDK, allowing for a more fully integrated experience when developing Silverlight apps. One difference between the Silverlight client library and the .NET and AJAX client libraries is that Silverlight 2 does not support synchronous development. Therefore, development with the Silverlight client library must make use of the asynchronous APIs, following the common begin/end async pattern. This also means that some of the APIs that you might use in the other two client libraries haven’t been enabled in the Silverlight client library. AJAX Client Library The ASP.NET AJAX library is currently available at codeplex.com/ aspnet/Release/ProjectReleases.aspx?ReleaseId=13357. Similar to the .NET Figure 7 Delay-Loading Related Entities using LoadProperty // get a single category DataServiceQuery categories = context.CreateQuery ("/Categories(1)"); foreach (Categories c in categories) { Console.WriteLine(c.CategoryName); context.LoadProperty(c, "Products"); foreach (Products p in c.Products) { Console.WriteLine("\t" + p.ProductName); } When using LINQ to ADO.NET Data Services, the set of queries expressible in the LINQ syntax is broader than those enabled by the REST-based URI syntax of Data Services. If a query cannot be mapped to a valid URI in the target data service, an exception will be thrown. One way to understand the types of queries that can and cannot be mapped to URIs is to think about hierarchical traversal. Any query that requires two or more pivots, (for example, joins or subqueries that use clauses such as Any), cannot currently be mapped to a URI. Queries with a single pivot and association traversals, however, generally work well. So far we have queried simple entities. Associations between objects are also tracked and managed by the DataServiceContext and associated objects can be loaded eagerly or as needed, using the URL formats discussed in the Uniform URI Formats section. To load associated entities on an as-needed basis (delay loading), use the LoadProperty method on the DataServiceContext class, as shown in Figure 7. In some scenarios, we may also want to avoid the extra round-trip on the wire to fetch the entities, preferring to load them along with the original query. In this case, the expand option is specified on the URI. The client library recognizes that the result includes both top-level and associated entities and will materialize all of them as a single object graph. Figure 8 eagerly loads related products in a single round-trip to the data service. Command Mapping } With queries through ADO.NET Data Services mapped through HTTP Get requests, how are Create, Update, and Delete requests executed? Each of the four CRUD operations (Create, Retrieve, Update, Delete) are mapped to a different HTTP verb with Retrieve mapped to GET, Create mapped to POST, Update mapped ADO.NET Data Services 56 msdn magazine http://codeplex.com http://codeplex.com/aspnet/Release/ProjectReleases.aspx?ReleaseId=13357 http://codeplex.com/aspnet/Release/ProjectReleases.aspx?ReleaseId=13357

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

MSDN Magazine - August 2008 Toolbox CLR Inside Out Basic Instincts Cutting Edge Patterns in Practice Data 2.0 - Expose And Consume Data In A Web Services World Biztalk EDI - Build A Robust EDI Solution With BizTalk Server Silverlight - Create Animations With XAML And Expression Blend Write On! - Create Web Apps You Can Draw On With Silverlight 2 Wicked Code - Craft Custom Controls For Silverlight 2 Team System Foundations Windows With C++ Concurrent Affairs Going Places { End Bracket }

MSDN Magazine - August 2008

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