Many times, when using a Silverlight application, I thinked about how cool would be to detach the application from the browser and use it directly from my desktop. Some Silverlight application are really lightweighted and can easily used on many platforms without to have to install it in the computer; the only thing needed is the Silverlight runtime.

This scenario I've described in a few words may open the way to a new big set of applications. Think as an example at an application that may be used both online and offline checking for network connectivity and synchronizing the modification of the local storage to a global online repository. You may detach multiple instances of the application in many computers, the office workstation, the home PC, your mini notebook and have them synchronized each other when you apply some changes. This may be a real occasionally connected application.

Out of browser experience and Silverlight 3.0

The biggest feature of the new Silverlight 3.0, called Out of browser experience, OOB for the friends, let you start creating application that can easily detached and used from a simple host without the need of a browser instance. The action of detaching the application is really simple, very fast and do not need any kind of installer. You have simply to right click the running application and if it is configured correctly you will find an "Install ... onto this computer..." item. The only question you need to answer is where you want silverlight let create the icon, on the Desktop or in the Start menu (or both) and then your OOB will be started and ready to be used.

The use of this kind of experience need open some problem we will analize in the next paragraphs, as usual using a small project as a guide. I've build a very tiny ToDoList application in Silverlight to let manage some simple todo, with a title a description and an optional due date. The application is ready to detect network changes and to synchronize contents using a WCF service, but this part is not implemented because is out of the scope of this article.

Configuring the Out Of Browser Experience

The first problem we have to address after we have completed our development, is how to configure it to let the user detach the application to be able to run it online and offline without a browser. This is a quite simply task. In the silverlight project you have to find the AppManifest.xml file under the Properties folder and uncomment a special section. Obviously you may also customize some properties:

   1: <Deployment.ApplicationIdentity>
   2:   <ApplicationIdentity ShortName="ToDoList"
   3:                        Title="Silverlight ToDoList">
   4:     <ApplicationIdentity.Blurb>
   5:       Manage offline your tasks
   6:     </ApplicationIdentity.Blurb>
   7:   </ApplicationIdentity>
   8: </Deployment.ApplicationIdentity>

The properties you can customize are the ShortName that will appear on the menu item, the title and the description (blurb) that will be displayed in the installation window. You may also add an optional <ApplicationIdentity.Icons> section. This let you add some icons in png format to be displayed on the desktop or start menu.

The installation window let you decide how to install  the application you are detaching. You may choice to add some icons, one on the destop and one on the Start menu.

This is the only action you need to perform. If you click yes the files required to run the application will be copied into a folder in AppData and the icons wil be created as shortcut to the sllauncher.exe into the Silverlight 3.0 install location.

The folder where the application files will be copied is C:\User\...\AppData\LocalLow\Microsoft\Silverlight\Offline.

After installing the OOB application you may run it again double clicking on the icons. You may also uninstall it simply right clicking on the running instance and selecting "Remove this application..." from the context menu.

The installation process may be triggered programmatically using the Detach() method of the Application class. This the the developer to add a button that indicate to the user that the application may be detached. The Application.RunningOffline property let the developer know if the Application is already detached. Using this property we may show the installation instructions when the application is in the browser and the remote them when installed.

If you imagine to have a "detach" button you can write this code:

   1: this.detach.Visibility = 
   2:     Application.Current.RunningOffline ? Visibility.Collapsed : Visibility.Visible;
   4: this.detach.Click += (sender, e) => Application.Current.Detach();

We may get information about the installation (detaching) or disinstallation of the application handling the event ExecutionStateChanged and reading the ExecutionState property in the Application.Current instance. This property let us know that the state of the application is:

  • Detached - The application has successful detached
  • DetachedUpdatesAvailable - see the paragraph (Updating the application)
  • DetachFailed - The detach is failed
  • Detaching - The detach is in progress
  • RunningOnline - The application is into the browser
Store local data: Isolated Storage

Running an Out of browser application imply to have a location where to store some data when the application is Offline. Unfortunately there is no way to access the local filesystem, also if the application is Detached by the user and also there is not any way to consume some kind of database (e.g. a Sql Server Mobile file).

The only place you can use to persist information is the local IsolatedStorage. The example application attached to this article use the IsolatedStorage to persiste a simple XML file containing the Tasks and the actions the user apply to them. This XML is loaded at the application startup and saved to the storage every time the user modify the data. To handle the synchronization of the modification to the online store the XML contains a status and version for each Task. This information may be used by the WCF Service to manage the updating of the online store.

Detecting network changes

After our OOB application is ready and works against the isolated storage we needs to detect when the user go online to trigger the syncronization with the online storage. The sncronization is made of two parts: we need to update the online store with the updates applied locally and the we needs to download locally the updates applied by other instances of the OOB application.

Detecting the network changes is sligthly simple. We may use two classes called NetworkChange and NetworkInterface. The first class expose and event that let us known when the network status has changed from online to offline and viceversa:

   1: NetworkChange.NetworkAddressChanged +=
   2:     new NetworkAddressChangedEventHandler(NetworkChange_NetworkAddressChanged);
   4: // ... omissis ...
   6: private void NetworkChange_NetworkAddressChanged(object sender, EventArgs e)
   7: {    
   8:     if (NetworkInterface.GetIsNetworkAvailable() && this.NeedUpdate)
   9:     {
  11:     }
  12: }

The events triggered by the NetworkChange class do not contains any informations about the connection is up or down. This event let us to know only that something has changed. Also we have to take note that the event will be triggered multiple times depending how many adapters are connected to the host computer so we have to handle this event accurately.

To detect if we are are running online or offline we may use the GetIsNetworkAvailable() method. This method returns true if this ia any connected network else it returns false. In the application we check also the NeedUpdate property to avoid that the update process will be triggered too many times if the network connect or disconnect many times due to network instability. In this application NeedUpdate returns true only if at least 1 hour has passed after the last successful update.

Updating the application

When an application is running online every time we connect to the host site we get the latest version of the application. The silverlight plugin handles the updates of the xap downloading the file and discarting the cached version. When an application has ben taken out of browser we needs to have this check to run sometimes because we want our applicatio remain up to date.

The Silverlight 3.0 OOB application run this check every time the application starts on the host computer. If the network is available and an update has been detected the new xap will be downloaded and then the running application will be notified of the available update. It is in charge of the developer to decide to show to the user that the application has been updated. However when the application is closed and restarted the update will be applied. Here is the code to get the notification about updates:

   1: Application.Current.ExecutionStateChanged += 
   2:     new EventHandler(Current_ExecutionStateChanged);
   4: // ... omissis ...
   6: void Current_ExecutionStateChanged(object sender, EventArgs e)
   7: {
   8:     if (Application.Current.ExecutionState == ExecutionStates.DetachedUpdatesAvailable)
   9:         MessageBox.Show("Updates have been downloaded. Please restart the application to apply this updates.");
  10: }

There is no way to break the update process. When the application detect an update it try to download the new application and updates itself. I something fail (e.g. the network is disconnected), the update process restarts the next time the application run.

Running the sample

Attached to this article you may found a Visual Studio 2008 project that shows a real application using the Out Of Browser Experience. To run this sample you need to have the Silverlight 3.0 installed and the Silverlight Tools for Visual Studio 2008 updated to the latest version. You have to know that after installing this updates the computer will be a full Silverlight 3.0 machine so if you have to develope something with Silverlight 2.0 it is better you not install anything.

I think the OOB experience will trigger many new applications. The simple ToDoList I've created is the evidence that unleashing your fantasy you may think about many applications of this tecnique. In the next month I hope many of you will stars publish your own sample.

Download code: (2.02 MB)

Demo Video: (3.5 MB)

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