public void DeregisterClient(IServiceAEvents client)
 {
     if (mCallbackList.Contains(client))
     {
         mCallbackList.Remove(client);
         Console.WriteLine("Successfully removed client: " + client);
     }
 }
        public void RegisterClient()
        {
            IServiceAEvents newClient =
                OperationContext.Current.GetCallbackChannel <IServiceAEvents>();

            if (!mCallbackList.Contains(newClient))
            {
                mCallbackList.Add(newClient);
                Console.WriteLine("Successfully added client: " + newClient);
            }
        }