MSDN Magazine - January 2009 - (Page 34)

OpenWriteAsync for uploading any data to a URL using a stream. You can use the Headers property to specify additional headers because the class, by default, doesn’t specify any. You should note, though, that some of the headers you may set are stripped off by the Framework and managed internally. These include Referer, Connection, and User-Agent. The Content-Type header, if set, is preserved. When it comes to downloading a resource, the WebClient class transparently uses the browser’s cache before attempting a connection. If the XAML or XAP resource is not in the cache, then the class proceeds with the download. The process of downloading content from a Silverlight application results from the combined efforts of the Silverlight runtime and the internal API that the host browser exposes to a plug-in. This means that under the hood of WebClient, the Silverlight runtime talks to the browser sandbox to check whether the requested resource is already in the cache. If not, Silverlight proceeds with its own security policies to authorize the request. When data is finally returned from the endpoint, the Silverlight runtime caches it locally through the browser’s services and in full respect of ongoing caching policies. A number of security restrictions apply to the WebClient Silverlight currently supports zones only on a Windows operating system. class and other HTTP classes in the Silverlight System.Net namespace. In particular, the WebClient class supports only HTTP and HTTPS schemes when downloading via stream and the FILE scheme when downloading pure XAML. Cross-scheme access is always prohibited, so you can’t point WebClient to, say, an HTTPS resource if the host page downloaded via HTTP (and vice versa). A WebClient request can, in general, reach a URL in a different browser’s zone, but not if it attempts to move from an Internet zone to a more restrictive zone. Silverlight currently supports zones only on a Windows operating system. Finally, cross-domain access is supported only if the remote site opted in by hosting a proper XML file in its root directory. Note also that cross-domain access doesn’t work in an HTTPS-to-HTTPS scenario. The same WebClient object can’t handle multiple requests simultaneously. You should check the IsBusy property—a Boolean value—to determine whether it is safe for your code to place a new request via the same WebClient instance. If you use multiple WebClient objects—perhaps on different threads—you can start multiple downloads at the same time. Downloading XAML-Only Data Let’s see how to use the WebClient to download XAML data and integrate it in the visual tree. In Silverlight 2 applications, the dynamic download of plain XAML data doesn’t necessarily give you the programming power you need. The XAML string must be plain XAML with no references to be resolved at run time ,such as bindings or references to styles, resources, and events. Once the XAML string is downloaded, you use the XamlReader class to turn it into a UI element that can be added to the existing document object model. The following code shows how to programmatically download a XAML string from a URL. Note that you need to provide the URL as a Uri object: WebClient client = new WebClient(); client.DownloadStringCompleted += new DownloadStringCompletedEventHandler(OnDownloadCompleted); Uri uri = new Uri("xaml.ashx", UriKind.Relative); client.DownloadStringAsync(uri); The URL can point to a plain XAML resource or to an endpoint that returns a text/xaml response. Here’s the code to process the downloaded XAML and append it to a placeholder in the visual tree: void OnDownloadCompleted(object sender, DownloadStringCompletedEventArgs e) { // Parse XAML to a UI element string xaml = e.Result; UIElement dom = XamlReader.Load(xaml) as UIElement; // Append to the DOM Placeholder.Children.Clear(); Placeholder.Children.Add(dom); } As mentioned, a placeholder may be any element in the document object model currently rendered in the plug-in. Note that the children of a UI element form a collection, and they are ren34 msdn magazine Cutting Edge http://www.ektron.com/MSDN http://www.ektron.com/MSDN

Table of Contents Feed for the Digital Edition of MSDN Magazine - January 2009

Toolbox CLR Inside Out Basic Instincts Cutting Edge Test Run First Look� Geneva Framework Silverlight Windows Mobile Service Station Security Briefs Extreme ASP.NET Foundations .NET Matters { End Bracket }

MSDN Magazine - January 2009

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