problem in web service with Silverlight

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Silverlight

hi everyone i have problem in web service with Silverlight,
i created web service with hello world and added this reference to my Silverlight apps in vs 2010 and write this code.

private void button1_Click(object sender, RoutedEventArgs e)
       {
           //ServiceReference1.WebService1SoapClient s = new WebService1SoapClient();

           //s.HelloWorldAsync();
           //s.HelloWorldCompleted +=new EventHandler<helloworldcompletedeventargs>(s_HelloWorldCompleted);

           WebService1SoapClient s = new WebService1SoapClient();
         s.HelloWorldCompleted +=new EventHandler<helloworldcompletedeventargs>(s_HelloWorldCompleted);
         s.HelloWorldAsync();


       }
      void s_HelloWorldCompleted(object sender, HelloWorldCompletedEventArgs  e)
       {
           if (e.Error != null)
           {
               textBox1 .Text =e.error.Message;
           }
           else
           {
               textBox1 .Text = "Number of users: " + e.Result;
           }
       } </helloworldcompletedeventargs></helloworldcompletedeventargs>

and when run i have error

An error occurred while trying to make a request to URI 'http://ws.mywebsite.com/WebService1.asmx'. This could be due to attempting to access a service in a cross-domain way without a proper cross-domain policy in place, or a policy that is unsuitable for SOAP services. You may need to contact the owner of the service to publish a cross-domain policy file and to ensure it allows SOAP-related HTTP headers to be sent. This error may also be caused by using internal types in the web service proxy without using the InternalsVisibleToAttribute attribute. Please see the inner exception for more details.

thanks for any help