Silverlight HTTPRequest thread problem

Question

I use the following code for handling an async response:

        private static void RespCallback(IAsyncResult ar)
        {
            // Get the RequestState object from the async result.
            RequestState rs = (RequestState)ar.AsyncState;

            // Get the WebRequest from RequestState.
            WebRequest req = rs.Request;

            // Call EndGetResponse, which produces the WebResponse object
            //  that came from the request issued above.
            WebResponse resp = req.EndGetResponse(ar);

            //  Start reading data from the response stream.
            Stream ResponseStream = resp.GetResponseStream();

            // Store the response stream in RequestState to read 
            // the stream asynchronously.
            rs.ResponseStream = ResponseStream;

            //  Pass rs.BufferRead to BeginRead. Read data into rs.BufferRead
            IAsyncResult iarRead = ResponseStream.BeginRead(rs.BufferRead, 0,
               BUFFER_SIZE, new AsyncCallback(ReadCallBack), rs);
        }


        private static void ReadCallBack(IAsyncResult asyncResult)
        {
            // Get the RequestState object from AsyncResult.
            RequestState rs = (RequestState)asyncResult.AsyncState;

            // Retrieve the ResponseStream that was set in RespCallback. 
            Stream responseStream = rs.ResponseStream;

            // Read rs.BufferRead to verify that it contains data. 
            int read = responseStream.EndRead(asyncResult);
            if (read > 0)
            {
                // Prepare a Char array buffer for converting to Unicode.
                Char[] charBuffer = new Char[BUFFER_SIZE];

                // Convert byte stream to Char array and then to String.
                // len contains the number of characters converted to Unicode.
                int len =
                   rs.StreamDecode.GetChars(rs.BufferRead, 0, read, charBuffer, 0);

                String str = new String(charBuffer, 0, len);

                // Append the recently read data to the RequestData stringbuilder
                // object contained in RequestState.
                rs.RequestData.Append(
                   Encoding.Unicode.GetString(rs.BufferRead, 0, read));

                // Continue reading data until 
                // responseStream.EndRead returns –1.
                IAsyncResult ar = responseStream.BeginRead(
                   rs.BufferRead, 0, BUFFER_SIZE,
                   new AsyncCallback(ReadCallBack), rs);
            }
            else
            {
                if (rs.RequestData.Length > 0)
                {
                    //  Display data to the console.
                    string strContent;
                    strContent = rs.RequestData.ToString();

                }
                // Close down the response stream.
                responseStream.Close();
                // Set the ManualResetEvent so the main thread can exit.
                allDone.Set();
            }
            return;
        }

I got this code from MSDN.

I am trying to get a response using the following code:

            System.Uri uri = new Uri("http://www.contoso.com");
            WebRequest wreq = WebRequest.Create(uri);

            // Create the state object.
            RequestState rs = new RequestState();

            // Put the request into the state object so it can be passed around.
            rs.Request = wreq;

            // Issue the async request.
            IAsyncResult r = (IAsyncResult)wreq.BeginGetResponse(
               new AsyncCallback(RespCallback), rs);
            allDone.WaitOne();

But application stops on allDone.WaitOne();. When I comment this string application works correct and is getting a response. What is wrong with request asynch thread?