public void GetLatestTransactionDate(Action<DateTime, Exception> callback)
            {
                try
                {
                    var client = new MainClient();
                    client.GetLatestTransactionDateAsync();

                    client.GetLatestTransactionDateCompleted += (sender, e) =>
                    {
                        if (e.Error == null)
                            callback(e.Result, null);
                        else
                            callback(DateTime.Now, e.Error);
                    };
                }
                catch (Exception ex)
                {
                    callback(DateTime.Now, ex);
                }
            }