MSDN Magazine - February 2009 - (Page 36)

Figure 7 Loading All Assemblies in the Manifest private Assembly GetAssemblyFromPackage( string assemblyName, Stream xapStream) { // Local variables StreamResourceInfo resPackage = null; StreamResourceInfo resAssembly = null; // Initialize Uri assemblyUri = new Uri(assemblyName, UriKind.Relative); resPackage = new StreamResourceInfo(xapStream, null); resAssembly = Application.GetResourceStream( resPackage, assemblyUri); // Extract the primary assembly and load into the AppDomain AssemblyPart part = new AssemblyPart(); Assembly a = part.Load(resAssembly.Stream); // Load other assemblies (dependencies) from manifest Uri manifestUri = new Uri("AppManifest.xaml", UriKind.Relative); Stream manifestStream = Application.GetResourceStream( resPackage, manifestUri).Stream; string manifest = new StreamReader(manifestStream).ReadToEnd(); // Parse the manifest to get the list of referenced assemblies List parts = ManifestHelper.GetDeploymentParts(manifest); foreach (AssemblyPart ap in parts) { // Skip over primary assembly (already processed) if (!ap.Source.ToLower().Equals(assemblyName)) { StreamResourceInfo sri = null; sri = Application.GetResourceStream( resPackage, new Uri(ap.Source, UriKind.Relative)); ap.Load(sri.Stream); } } // Close stream and returns xapStream.Close(); return a; storage files for the application and delete them manually through Windows Explorer. Let’s see what it would take to add a simple expiration policy that makes any cached XAP file obsolete after a given amount of time after download. The proper place to set an expiration policy is the DownloadCache class, as in Figure 6. When you add a XAP file to the cache, you need to store some information about the download time. When you access the cache to pick up a package, you should verify whether the package is expired (see Figure 9). The implementation of such an apparently simple algorithm is hindered by one notable fact. In Silverlight, you have no way to access file attributes like last update or creation time. This means that you are responsible for managing time information. In other words, when you add a XAP to the cache you also create an entry Figure 8 Parsing the Manifest Using LINQ-to-XML public class ManifestHelper { public static List GetDeploymentParts(string manifest) { XElement deploymentRoot = XDocument.Parse(manifest).Root; List parts = (from n in deploymentRoot.Descendants().Elements() select new AssemblyPart() { Source = n.Attribute("Source").Value } ).ToList(); } return parts; } } Figure 9 Test for Expiration public bool IsExpired(string xapFile) { bool expired = true; if (m_ItemsIndex.ContainsKey(xapFile)) { DateTime dt = (DateTime)m_ItemsIndex[xapFile]; // Expires after 1 hour expired = dt.AddSeconds(3600) < DateTime.Now; if (expired) Remove(xapFile); ployed assemblies, and process them. Figure 7 shows how to load into memory all assemblies referenced in the manifest file. The manifest file is an XML file, as shown here: } } return expired; To parse this file, you can use LINQ-to-XML. The source code contains a sample ManifestHelper class with a method that returns a list of AssemblyPart objects (see Figure 8). It is worth noting that in Beta versions of Silverlight 2, you could use the XamlReader class to parse the manifest file to a Deployment object: // This code throws in Silverlight 2 RTM Deployment deploy = XamlReader.Load(manifest) as Deployment; Figure 10 Persisting Download Details to Isolated Storage public static Stream Load(string file) { IsolatedStorageFile iso; iso = IsolatedStorageFile.GetUserStoreForApplication(); if (!iso.FileExists(file)) { iso.Dispose(); return null; } // Check some expiration policy CacheIndex m_Index = new CacheIndex(); if (!m_Index.IsExpired(file)) return iso.OpenFile(file, FileMode.Open); // Force reload iso.Dispose(); return null; In the release version, the Deployment object has been transformed into a singleton and subsequently it can’t be used to load assemblies dynamically. The XML in the manifest therefore must be parsed manually. Expiration Policies As implemented thus far, the assembly cache is permanent and there’s no way for the user to get updated packages. The only possible workaround is to have a machine administrator locate isolated 36 msdn magazine } Cutting Edge

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

MSDN Magazine - February 2009 Contents Toolbox CLR Inside Out Data Points Cutting Edge Patterns In Practice Best Practices .Net Interop "Oslo" Basics Patterns Silverlight Under The Table Foundations Windows With C++ .NET Matters Going Places { End Bracket }

MSDN Magazine - February 2009

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