private void RefreshWorker_DoWork(object sender, DoWorkEventArgs e)
        {
            // retrieve id argument
            string id = e.Argument as string;

            // call twitter api
            TwitterClient  client         = new TwitterClient(_username, _password);
            DirectMessages directMessages = null;

            try
            {
                if (string.IsNullOrEmpty(id))
                {
                    directMessages = client.DirectMessages();
                }
                else
                {
                    directMessages = client.DirectMessages(id);
                }
                client.Close();
                e.Result = new DirectMessagesResult {
                    Id = id, DirectMessages = directMessages
                };
            }
            finally
            {
                client.Abort();
            }
        }