private void btnCalcRate_Click(object sender, RoutedEventArgs e) { CalcRate origin = (CalcRate)cb_ActiveCountryRate.SelectedItem; CalcRate destination = (CalcRate)cb_AllcountriesRate.SelectedItem; if (origin != null && destination != null) { if (string.IsNullOrWhiteSpace(tb_insertRate.Text)) { MessageBox.Show("Insert a value to convert"); return; } string input = tb_insertRate.Text; if (Regex.IsMatch(input, @"^-?[0-9][0-9,\.]+$")) { tb_rateResult.Text = $"{tb_insertRate.Text} {origin} correspond to: {Environment.NewLine} " + $" {Convert.ToString(decimal.Round(Convert.ToDecimal(UIData.CalculateRate(origin, destination, Convert.ToDecimal(input))), 2))} {destination.Name}"; } else { MessageBox.Show("The value to be converted must be numeric"); return; } } else { MessageBox.Show("You have to choose currencies to convert"); return; } }