Silverlight 4 Project Types part I – Silverlight Application

Ok, let’s take a look at the different types of Silverlight applications that you can create out-of-the-box with Visual Studio 2010.  Now that I’ve installed Visual Studio 2010 and Silverlight 4, I want to take a quick spin through each of the different project types that the New Project wizard makes available.

After firing up Visual Studio 2010, I click on the New Project link and then select the Silverlight project type under C#.  Here’s the default list that I see:

  • Silverlight Application
  • Silverlight Navigation Application
  • Silverlight Class Library
  • Silverlight Business Application
  • WCF RIA Services Class Library
  • Silverlight Unit Test Application

I’d like to take a quick look at each project type, looking at the pieces that make up the project and then thinking a little bit about when to use that particular project type.  As I walk through the various project types, I’d also like to think a little bit about the ecosystem in which the Silverlight application lives.  In other words, what other components might you typically create when designing a solution of that type?

I also want to think a little bit about the tooling.  What tools would you typically use to work on the project that you’re creating?  Would Visual Studio 2010 alone be sufficient?  Or would you spend a lot of time working in Blend?  What about SQL Server 2008?

The Silverlight Application

The first project type—the Silverlight Application—looks like it might be the same thing that I created with Visual Studio 2008 and Silverlight 2, back in the Hello Silverlight World, Part 1 post.

The New Application dialog that we see next is very similar to what I saw with Visual Studio 2008 and Silverlight 2.  As before, we have to host our Silverlight application somewhere.

Notice that this time we have one additional project type that we didn’t have before—an ASP.NET MVC Web Project.  So we can basically go with a “classic” ASP.NET web site or an MVC web site.

The default is to host Silverlight in a new ASP.NET web application project.  But if I uncheck the first checkbox, I get a simple Silverlight project with no containing web site:

If you go this route and then press F5 to “run” your Silverlight application, Visual Studio will just create a test .html page in which to host your Silverlight control.

If you take a look at this page, you’ll see a simple HTML page with a familiar <object> tag hosting the Silverlight control.  (See my post on the Lifecycle of a Silverlight Control for more details on how this is done).

If we instead go with the defaults on the New Silverlight Application dialog (host the Silverlight Application in a new ASP.NET Web Application project), we end up with a simple ASP.NET project that contains both .html and .aspx pages hosting the new Silverlight control.

In either case, .html or .aspx, your Silverlight application is embedded in the web page

<div id="silverlightControlHost">
 <object data="data:application/x-silverlight-2," type="application/x-silverlight-2" width="100%" height="100%">
 <param name="source" value="ClientBin/SLApp1.xap"/>
 <param name="onError" value="onSilverlightError" />
 <param name="background" value="white" />
 <param name="minRuntimeVersion" value="4.0.41108.0" />
 <param name="autoUpgrade" value="true" />
 <a href="" style="text-decoration:none">
 <img src="" alt="Get Microsoft Silverlight" style="border-style:none"/>
 </object><iframe id="_sl_historyFrame" style="visibility:hidden;height:0px;width:0px;border:0px"></iframe></div>

As I describedin The Lifecycle of a Silverlight Control, this <div> tag embeds your Silverlight application (control) into a web page using the application/x-silverlight-2 MIME type.  This MIME type now maps to the AgControl.AgControl.4.0 class, implemented in npctrl.dll, which can be found in C:\Program Files\Microsoft Silverlight\4.0.41108.0.

Visually, the pieces of the puzzle look like this (from MSDN article on Silverlight 4.0 Application Services):

So as before, a Silverlight application is managed code that runs in the context of a browser plugin.  Here’s how things work when your browser renders a page containing a Silverlight control:

  • Browser renders HTML  from .html page (or generated HTML from ASP.NET page)
  • Browser sees <object> tag for application/x-silverlight-2 MIME type and fires up the Silverlight plug-in
  • Silverlight plug-in loads the Silverlight core services
  • Silverlight plug-in loads the Silverlight CLR, which creates an AppDomain for your application
  • .xap file containing your Silverlight application is downloaded from the server
  • Your application manifest is loaded/read (from AppManifest.xaml, in .xap)
  • Your main Silverlight application DLL is loaded (from .xap)
  • Silverlight CLR instantiates your Application object and fires its Startup event
  • Your application’s Startup event creates a new instance of your main page (a Silverlight UserControl)
  • Your main page (e.g. MainPage in wizard-generated project) renders itself by reading/loading its .xaml file

Voila.  At the end of all this, you get a Silverlight Application rendered in an ASP.NET web page.  In our case, having generated everything using the Project Wizard, we can just press F5 to run the application–launching a browser window and loading your auto-generated ASP.NET page.  It looks like this (the Silverlight application has the green background):

The XAP Payload

One final topic to cover quickly is–what exactly does the downloaded .xap file contain?  In our wizard-generated test project, you’ll find a ClientBin directory, which contains the compiled Silverlight application in a .xap file.  In our case, it’s named SLApp1.xap.

If you just rename the .xap file as and double-click on it, you can see what it contains:

The main pieces here are the application manifest, which tells the Silverlight runtime what object to start up first, and the assembly in which that object lives.  The .xap file also contains the assembly (SLApp1.dll) that contains our actual code.

The .xap file also contains some dependent DLLs, which are sent down to the client as part of the .xap package.

Wrapping Up

That’s all there is to a basic Silverlight Application.  Next time, I’ll take a look at the Silverlight Navigation Application, which lets us create a complete Silverlight application with multiple pages, rather than a single user control.

Hello Silverlight World, part 2 – The Application Object

In my last Silverlight post, Hello Silverlight part 1 – Generating the Project, I started creating a Silverlight-based Hello World application.  I described the various pieces that get created by the Project Wizard in Visual Studio 2008.  I also started comparing the Silverlight application to a wizard-generated WPF application.

Let me continue looking at the individual pieces that make up a bare-bones Silverlight application.  Maybe the convention should be to call it a “Silverlight control”, since that seems a bit more accurate.  For now, I’ll stick with Visual Studio’s terminology and call it a “Silverlight application”.


Like our WPF application, the main entry point of the Silverlight application is described by the App.xaml / App.xaml.cs files.  In both WPF and Silverlight, these files define a subclass of System.Windows.Application, which serves as the main object loaded at runtime.

In both WPF and Silverlight, we can look at the code in App.xaml.cs and App.g.cs (generated at build time) to get a sense of how the application gets loaded and run.  What we see is very different behavior, despite the fact that both App classes appear to derive from System.Windows.Application.

In the WPF application, here’s what happens:

  • There is an entry point named Main()
  • Main()
    • Creates an instance of the App class
    • Calls the InitializeComponent method of the App object
    • Calls the Run method of the App object
  • App.InitializeComponent
    • Sets App.StartupUri property to point to a Uri object pointing to Window1.xaml, our main window

In the Silverlight application, something very different is going on at startup:

  • There is no Main() method
  • The App constructor
    • Wires up the Application_Startup event handler to the App.Startup property
    • Wires up the Application_Exit event handler to the App.Exit property
    • Wires up the Application_UnhandledException event handler to the UnhandledException property
    • Invokes InitializeComponent
  • Application_Startup
    • Instantiates a new Page object (the class for our main page)
    • Sets App.RootVisual to point to this page object
  • InitializeComponent
    • Calls Application.LoadComponent, passing it a new Uri object pointing to App.xaml

Why is the startup behavior so radically different, between WPF and Silverlight, given that the main application appears to be inherited from System.Windows.Application in both cases?

The answer is that, while the class name and namespace are identical, the Application object in Silverlight is a very different animal than the one that is part of WPF.  Remember that the WPF application is running against an entirely different .NET Framework than the Silverlight application.  WPF is referencing/using the full .NET Framework 3.5 (with pieces from 2.0, 3.0 and 3.5), while the Silverlight application is referencing/using the Silverlight Framework, which is an entirely different set of libraries.  While much of the Silverlight framework is a subset of the WPF framework, the Application object is entirely different.

  • In WPF, the System.Windows.Application class lives in PresentationFramework.dll
  • In Silverlight, System.Windows.Application lives in System.Windows.dll

PresentationFramework.dll is part of the .NET Framework 3.0 and exists on a  client system when they install the full .NET Framework (3.0 or 3.5).  System.Windows.dll is part of the Silverlight framework and exists on a client system when they install Silverlight.

You can see this by reading the documentation (MSDN).  Or you can see how different these Application classes are by doing the following:

  • In the WPF application, open the App.xaml.cs file, right-click on Application and choose Go To Definition
    • You’ll see that the Application class has a Run method and a StartupUri property, but no RootVisual property
    • Now hover over the tab for this window in Visual Studio–you’ll see a temp filename that contains the name “PresentationFramework.dll”
  • In the Silverlight application, open the App.xaml.cs file, right-click on Application and choose Go To Definition
    • You’ll see a very different Application class, with a RootVisual property, but no Run method
    • Hover over the tab again–you’ll see a temp file name containing “System.Windows.dll”

Let’s Stop Here

That’s a good stopping point, for now.  We’ve looked just a little bit under the covers in the Silverlight application–enough to see that the main Application object loaded at runtime is very different from the object loaded in the WPF application.