MSDN Magazine - August 2008 - (Page 54)

Figure 5 Access an ADO.NET Data Service from a .NET App namespace TestApplication { public class Region { public int RegionID { get; set; } public string RegionDescription { get; set; } } class Program { static void Main(string[] args) { DataServiceContext ctx = new DataServiceContext("http://localhost:25115/NorthwindService.svc"); DataServiceQuery regions = ctx.CreateQuery ("/Region?$orderby=RegionID"); foreach (Region r in regions) { Console.WriteLine(r.RegionID + ", " + r.RegionDescription); } } } } Figure 6 Add Service Reference deletes as change interceptors. Change interceptors have no return value and take two arguments, an object of the type contained in the EntitySet and an UpdateOperations enumeration that defines the action (update, insert, or delete) being requested on the resource. The interceptor may alter the object passed to it or set the reference to an entirely different instance. If the method throws an exception, the operation is aborted, no change is made in the database, and an error is returned to the client agent. Client Side: Accessing the Service is further simplified using the ADO.NET Data Services Client Libraries. This presents a more natural programming model for applications written using the .NET Framework, Silverlight, and ASP.NET AJAX to target data services, manipulate the results in terms of objects, and handle aspects such as association traversal. The client libraries abstract away the details of HTTP requests and responses and also ensure a more uniform experience across each of the client stacks. The .NET Framework and Silverlight client libraries consist of two primary types, the DataServiceContext class and the DataServiceQuery class. DataServiceContext represents the runtime context for a given data service. Data services themselves are stateless; however, the context in which the developer interacts is not, and state on the client is maintained between interactions in order to support features such as identity resolution and optimistic concurrency. The DataServiceQuery object represents a specific query against the store defined using the URI syntax. To execute a query and obtain the results in the form of .NET objects, you simply enumerate over the query object. You can do so using the standard “foreach” construct in C# or “For Each” in Visual Basic®, for example. In order to use the entities defined in a data service as .NET objects on the client, corresponding classes need to be defined for the client application. There are three primary methods for accomplishing this. One option is to define them manually. Figure 5 shows a simple hand-written definition for the Region class and a short piece of code that executes a query against regions and prints their IDs and descriptions in the output. A more common approach is to use code generated by Visual Studio. While writing classes manually works well when the service has only a small number of types, as the data service schema grows more complex, the number and size of the classes that must be manually created and maintained also grows significantly. The most common way to generate the necessary classes is by using the Add Service Reference wizard in Visual Studio. Similar to using Add Service Reference for a WCF service, simply right-click on the Project and choose Add Service Reference (see Figure 6). In the Add Service Reference dialog, enter the URI for the entry point of the service as the Address. In our example, we would enter / /NorthwindService.svc. This will generate classes based on the data service definition and add them to the project. An alternate option is to use the “datasvcutil.exe” command-line tool to generate the classes based on the data service definition. This tool shipped with the release of ADO.NET Data Services and can be found in the directory \Windows\Microsoft.Net\Framework\ V3.5. The tool takes as an argument the URI for the data service’s entry point as well as the name and location of the output file that is to be generated, like so: "\Windows\Microsoft.Net\Framework\V3.5\datasvcutil.exe" /out:C:\NorthwindSample\northwind.cs /uri:"http:// / / NorthwindService.svc" In general, we have treated HTTP as the API for data services and thus have spent a lot of time ensuring that it is simple to use directly at the HTTP level. This means that not only is it easy to interpret service payloads, but that any platform with an HTTP stack can easily use data services. That said, if you are using a Microsoft platform to access data services, client-side development 54 msdn magazine The default output from the command-line tool is a C# file that contains a class for each of the entity types described in the data service (Visual Basic types can be generated using the /language:VB switch). Each generated class has members representing the primiADO.NET Data Services

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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