MSDN Magazine - November 2008 - (Page 25)

Figure 4 LINQ to XML Querying Digg Data XDocument xml = XDocument.Parse(rawXml); var storiesQuery = from story in xml.Descendants("story") select new DiggStory { Id = (int)story.Attribute("id"), Title = ((string)story.Element("title")).Trim(), Description = ((string)story.Element("description")).Trim(), ThumbNail = (story.Element("thumbnail") == null ? string.Empty : story.Element("thumbnail").Attribute("src").Value), Link = new Uri((string)story.Attribute("link")), DiggCount = (int)story.Attribute("diggs") }; Figure 5 Digg XML Response Microsoft has released SQL Server 2005 recently as an upgrade to the popular SQL Server database. Microsoft releases SQL Server 2005 Microsoft's virtual receptionist Microsoft's virtual receptionist Client class or the HttpWebRequest. Making calls through WebClient is simpler, but HttpWebRequest offers more control over how the requests are made. Also, WebClient returns on the UI thread while HttWebRequest returns on the background thread. HttpWebRequest’s callback must use the Dispatcher to interact with the UI. The following code shows the System.Net.WebClient retrieving recent stories from the Digg service: string baseUri = "http://services.digg.com/stories/topic"; string topic = txtTopic.Text; string appKey = "http%3A%2F%2Fwww.microsoft.com"; int count = int.Parse(txtTopicCount.Text); string url = String.Format("{0}/{1}?appkey={2}&count={3}", baseUri, topic, appKey, count); WebClient svc = new WebClient(); svc.DownloadStringAsync(new Uri(url)); Here the data is returned from the WebClient call as raw XML which can then be parsed or queried. If the REST service has parameters, they can be passed as part of the querystring. When a REST-based or POX service is invoked, it can return data as XML (REST services can also return data as JSON). The XML can then be parsed using a number of XML parsing libraries. However, the XML can also be queried using LINQ to XML, which offers a rich query interface to extract data from XML structures without having to resort to iterating through complex XML hierarchies. The REST service in the example you just saw returns raw XML from the Digg service that shows the most recent stories. This XML can then be parsed using an XML library or it can be queried using LINQ to XML, as shown in Figure 4. The XDocument class’s Parse method can consume the XML from the REST service shown earlier. The XML can then be queried using LINQ to XML. The code shown in Figure 4 demonstrates the LINQ to XML syntax for querying the XML shown in Figure 5. The query starts in the stories/story hierarchy and queries all of the story elements. There is no namespace specified for this XML; however, I left the namespace variable (ns) in place to show how to prefix all XML paths in case the results do refer to a namespace. Driving story elements is accomplished using the XDocument instance’s Descendants method. The story element’s properties can then be retrieved using the story variable and its Element method. For example, the following line of code grabs the value of the title element from the story element and sets it into a property named Title: Title = ((string)story.Element("title")).Trim() How do I query XML? The LINQ to XML query can return the results through a projection or into a class structure. The code shown in Figure 4 creates a DiggStory class to store the results of each status element retrieved from the XML data. The DiggStory class that the developer has defined, where all of the properties (for simplicity) have been defined as string. LINQ to XML could include other aspects such as ordering or filtering criteria, too, if needed. Using LINQ to XML to query and parse XML data from REST or POX services allows developers to leverage their existing knowledge of other LINQ forms since the syntax is very similar across them all. Perhaps the biggest benefit of using LINQ to XML to parse the data is the power and simplicity of querying the data without having to resort to iterating through elements in nested foreach loops. Do I need to worry about cross-domain policies when I’m accessing third-party Web services? When a Silverlight 2 application makes a Web service call, the call is precluded by a check on the Web service’s server for a cross-domain policy file. This check is done when the Silverlight application Silverlight 2 Application Hosted on microsoft.com Silverlight 2 Application Hosted on johnpapa.net Request Allowed D e n ie d Request microsoft.com johnpapa.net Request Allowed Figure 6 Cross-Domain Checking November 2008 25 msdnmagazine.com http://www.msdnmagazine.com

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

Contents MSDN Magazine - November 2008 Toolbox CLR Inside Out Data Points Cutting Edge Security Briefs Test Your Security IQ Agile SDL Access Control Utility Spotlight RIA Test Run Wicked Code Foundations Team System End Bracket

MSDN Magazine - November 2008

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