This really falls into the category of "just because it's there" because it's something I was thinking about earlier today and wanted to experiment with.

Silverlight 2 applications can make web service calls across domain boundaries if the domain that they are calling to allows that. So, I was thinking about a scenario where perhaps I've got a local application installed such as a WPF application or perhaps a Vista Sidebar gadget and I go and visit a Silverlight website and the Silverlight client can notify the installed application and cause it to maybe "light up" in some fashion.

That is, a Silverlight 2 application communicating with another application that's installed and running on the local machine.

Before Silverlight will allow a cross-domain call it requests one of two files from the root of the site that it's calling to. I chose to use clientaccesspolicy.xml.

So, I wrote a quick "WCF Service" using the System.ServiceModel.Web stuff from .NET Framework V3.5. Service looks like this;

[ServiceContract]
public interface IHandlePolicy
{
  [OperationContract]
  [WebInvoke(UriTemplate = "clientaccesspolicy.xml", Method="GET", BodyStyle=WebMessageBodyStyle.Bare)]
  XElement ReturnPolicy();
}

so if you request a URL clientaccesspolicy.xml then what I'll do is this;

public class PolicyHandler : IHandlePolicy
{
  public XElement ReturnPolicy()
  {
    return (XElement.Load("policy.xml"));
  }
}

and that policy.xml file simply looks like this;

<?xml version="1.0" encoding="utf-8" ?>
<access-policy>
  <cross-domain-access>
    <policy>
      <allow-from>
        <domain uri="*"/>
      </allow-from>
      <grant-to>
        <resource path="/"
                  include-subpaths="true"/>
      </grant-to>
    </policy>
  </cross-domain-access>
</access-policy>

I configured that up like this;

     <service name="PolicyHandler">
        <endpoint address="http://localhost:9091/"
                  binding="webHttpBinding"
                  contract="IHandlePolicy"
                  behaviorConfiguration="webHttpBehaviour"/>
      </service>

and with a behaviour like this;

      <endpointBehaviors>
        <behavior name="webHttpBehaviour">
          <webHttp/>
        </behavior>
      </endpointBehaviors>

and serving it up with code;

  static void Main(string[] args)
  {
    WebServiceHost webHost = new WebServiceHost(typeof(PolicyHandler));
    webHost.Open();

    Console.WriteLine("Listening...");
    Console.ReadLine();
    
    webHost.Close();
  }

So, now if I request URL http://localhost:9091/clientaccesspolicy.xml then this service will reply with my policy file.

I then added into the same project another SOAP based service. Aside - I was going to use a the System.ServiceModel.Web stuff here as well but then discovered that Silverlight does not seem to have the webHttpBinding or the WebInvoke/WebGet bits of .NET Framework V3.5 in the beta so I went with SOAP messages to a separate service listening on a sub path of the initial URI.

Here's the service;

[ServiceContract]
public interface IPrintMessages
{
  [OperationContract]
  void PrintMessage(string message);
}

and the implementation (not too exciting);

public class MessagePrinter : IPrintMessages
{
  public void PrintMessage(string message)
  {
    Console.WriteLine(message);
  }
}

and the configuration;

 <service name="MessagePrinter"
               behaviorConfiguration="metadataBehaviour">
        <endpoint address="http://localhost:9091/printer"
                  binding="basicHttpBinding"
                  contract="IPrintMessages"/>

<serviceBehaviors>
        <behavior name="metadataBehaviour">
          <serviceMetadata httpGetEnabled="true"
                           httpGetUrl="http://localhost:9091/printer"/>
        </behavior>
      </serviceBehaviors>

and, finally, the hosting code now looks like;

  static void Main(string[] args)
  {
    WebServiceHost webHost = new WebServiceHost(typeof(PolicyHandler));
    webHost.Open();

    ServiceHost soapHost = new ServiceHost(typeof(MessagePrinter));
    soapHost.Open();

    Console.WriteLine("Listening...");
    Console.ReadLine();

    soapHost.Close();
    webHost.Close();
  }

I built a quick Silverlight application with a TextBox and a Button, added a Service Reference to my web service by pointing at the WSDL file and then wrote a little code;

public partial class Page : UserControl
  {
    public Page()
    {
      InitializeComponent();
    }
    private void Button_Click(object sender, RoutedEventArgs e)
    {
      PrintMessagesClient proxy = new PrintMessagesClient();
      proxy.PrintMessageAsync(myText.Text);
    }
  }

and, hey presto, I can see Silverlight requesting my clientaccesspolicy.xml file at runtime and then calling my service and I have my Silverlight app talking to my local app;

Now...whether there's actually a good use for that is another question but it was fun to me for a little while :-)

I put the zipped up project files here in case you want to play with them.