HomeControllerAsync.cs

    public class HomeControllerAsync : AsyncController
    {
        /// depends on long running service call.
        public void FindAnswerAsync()
        {
              var wc = new WebClient();
              wc.DownloadStringCompleted += (s, e) => {
                 AsyncManager.Parameters["result"] = e.result;
                 AsyncManager.OutstandingOperations.Decrement();
              }
              AsyncManager.OutstandingOperations.Increment();
              wc.DownloadStringAsync(new Uri("some_url"));
        }

         /// Handles FindAnswer completion.
        public ActionResult FindAnswerCompleted(string result)
        {
            ViewBag.result = result;    
            return View();
        }
   }