private void CardNumber_Completed(object sender, System.EventArgs e) { if (CardNumber.Text.Length > 16 || CardNumber.Text.Length < 12) { ErrorLabel_CardNumber.IsVisible = true; ErrorLabel_CardNumber.Text = "Invalid card number"; EnableSubmitButton(); } else { ErrorLabel_CardNumber.IsVisible = false; } CardNumber.Unfocus(); Month.Focus(); EnableSubmitButton(); }