WCF service - how to invoke EndAsyncMethod at a certain time?

Dear Guru! 
Silverlight client accesses a wcf service with the task to report on the change that occurred in the mssql database. 
Obviously in ServiceAsyncMethod - must be run expectation interrupt mssql (via ServiceBroker). But what to do next? 
Here is a sample text of wcf:

        [OperationContractAttribute(AsyncPattern = true)]
        public IAsyncResult BeginServiceAsyncMethod(string msg, AsyncCallback callback, object asyncState)
        {
            AddEventChanges();
            emergEvent ev = new emergEvent();
            return new CompletedAsyncResult<emergEvent>(ev);
        }

        public emergEvent EndServiceAsyncMethod(IAsyncResult r)
        {
            CompletedAsyncResult<emergEvent> result = r as CompletedAsyncResult<emergEvent>;
            result.Data.TimeDetected = DateTime.Now;
            return result.Data;
        }

        public void AddEventChanges()
        {
           using (SqlConnection connection = new SqlConnection(connectionString))
            {
                using (SqlCommand cmd = new SqlCommand("Select id_event,name,description from dbo.events", connection))
                {
                    cmd.Notification = null;
                    SqlDependency dependency = new SqlDependency(cmd);
                    dependency.OnChange += new OnChangeEventHandler(WaitChange);
                    connection.Open();

                    using (SqlDataReader reader = cmd.ExecuteReader())
                    {
                    }
                }
            }
        }

        public void WaitChange(object sender, SqlNotificationEventArgs e)
        {
            string s = "Changing in database:";
            s += "\nInfo - " + e.Info.ToString() + "\nSource = " + e.Source.ToString() + "\nType - " + e.Type.ToString();
            MessageBox.Show(s);
            AddEventChanges();
        }

When a change occurs  the WaitChange procedure  is called. How to do that at this moment EndServiceAsyncMethod would be invoked?