Getting back on the UI thread in Silverlight

public class BingModel : IBingModel
{
    // this string is used to make the actual call to the bing API. 
    // the blancs are filled in when SearchBing method is called
    const string BingRequestURL =
        "http://api.bing.net/json.aspx?AppId={0}&Version=2.2&"
        +"Market=en-US&Query={1}&Sources=web&Web.Count={2}";
 
    // the request is kept so it can be used to handle the response
    private WebRequest request;
 
    // the SearchBing method is called from the UI thread
    //(note: //you can get your AppId at http://bing.com/developers)
    public void SearchBing(string searchText, string appId)
    {
        //the blancs are filled in  
        string requestString = 
            string.Format(BingRequestURL, 
                          appId, 
                          HttpUtility.UrlEncode(searchText),
                          20); //20 results will be givin.
        //a new request is made.
        request = HttpWebRequest.Create(requestString);
        
        //the getting of the response is started. The OnRequestCompleted 
        //method is provided so it can be called when the request is 
        //completed. The Current SynchronizationContext is provided as 
        state so it can be handled by the result.
        request.BeginGetResponse(OnRequestCompleted, 
                                 SynchronizationContext.Current);
    }
 
    //when the request is completed this method is called
    private void OnRequestCompleted(IAsyncResult ar)
    {
        //the response is extracted from the result and read thru a stream.
        var webResponse = (HttpWebResponse)request.EndGetResponse(ar);
        var response = new StreamReader(webResponse.GetResponseStream());
        
        //Because a string of Json is returned by the service, this string is 
        //parsed into a JsonObject.
        var json = JsonObject.Parse(response.ReadToEnd());
 
        //the ExtractResults method (defined below) is used to create 
        //an enumeration of the results
        IEnumerable<BingResult> ress = ExtractRestults(json);
 
        // this method (defined below) is used to fire a custom event on the UI thread
        // the state is provided as Synchronization context along with the json results
        InvokeSearchBingCompletedEvent(ar.AsyncState as SynchronizationContext, ress);
    }
    
    private void InvokeSearchBingCompletedEvent(SynchronizationContext context, 
                                                IEnumerable<BingResult> ress)
    {
        if (context != null) // just in case the context has become null somehow
            // call an anonymous method asynchronous on the context that was used to start 
            //the search
            context.Post((e) => 
                     {
                         if (SearchBingCompleted != null) 
                             //raise the event
                             SearchBingCompleted(this, new SearchBingCompletedArgs() 
                                                   {
                                                       SearchResults = ress.ToList()
                                                   }
                                 );
                     }, null);
    }
 
    // Extract search result from Json returned by Bing API  
    private static IEnumerable<BingResult> ExtractRestults(JsonValue json)
    {
        var results = json["SearchResponse"]["Web"]["Results"] as JsonArray;
 
        return from res in results
               select new BingResult()
                          {
                              Title = res["Title"],
                              Url = res["Url"]
                          };
    }
    
    // the definition of the event handler
    public event EventHandler<SearchBingCompletedArgs> SearchBingCompleted;
}