namespace wcf
{

    public class asyncResult : IAsyncResult{
        public string message = String.Empty;
        object state;
        public string result {get; set;}
        ManualResetEvent manualResentEvent;
        AsyncCallback callback;

        public asyncResult (string _msg, AsyncCallback callback, object state) {
            this.message = _msg;
            this.callback = callback;
            this.state = state;
            this.manualResentEvent = new ManualResetEvent(false);
         }

        object IAsyncResult.AsyncState { get { return state; } }
        public ManualResetEvent AsyncWait { get { return manualResentEvent; } }
        WaitHandle IAsyncResult.AsyncWaitHandle  { get { return this.AsyncWait; } }
        bool IAsyncResult.CompletedSynchronously { get { return false; } }
        bool IAsyncResult.IsCompleted { get { return manualResentEvent.WaitOne( 0 , false); } }
  
        public void Complete() {
            manualResentEvent.Set();
            if (callback != null)
                callback(this);
        }
    }
}

namespace wcf 
{   
    [ServiceContract]
    public class asyncPrinter {            
        [OperationContract(AsyncPattern = true)]
        public IAsyncResult BeginPrint(string msg, AsyncCallback callback, object state) {       
            asyncResult ar = new asyncResult(msg, callback, state);
            ThreadPool.QueueUserWorkItem(new WaitCallback (Callback), ar);
            return ar;
        }

        private void Callback(object state) {
            asyncResult aRslt = state as asyncResult;
            try { aRslt.result = format(aRslt.message); }
            finally  { aRslt.Complete(); }
        }

        private string format(string msg) {
            Thread.Sleep( 3000 );
            return "formatted string " + msg; 
        }

        public string EndPrint(IAsyncResult arslt) {
            string res = String.Empty;
            if (arslt != null)
            {
                using (asyncResult aRslt = arslt as asyncResult) {
                    if (aRslt == null)
                        throw new ArgumentNullException("IAsyncResult parameter is null.");

                    aRslt.AsyncWait.WaitOne();
                    res = aRslt.result;
                }
            }
            return res;
        }
    }
}