private void OnCurrencyChanged(object sender, SelectionChangedEventArgs e)
        {
            TextRate.Text = "";

            string currency = (string)ComboBoxCurrency.SelectedItem;

            if (currency == "USD")
            {
                return;
            }

            System.Windows.Threading.Dispatcher dispatcher = this.Dispatcher;

            ExchangeRates rates = this.serviceProvider.GetService(typeof(ExchangeRates)) as ExchangeRates;

            if (rates != null)
            {
                rates.Enqueue(currency);
            }

            UpdateRateText();
        }