private void button_Ok_Click(object sender, RoutedEventArgs e) { if (_ui_logic.CheckAmountFormValid(textBoxCalculate.Text) == false) { MessageBox.Show("The number is incorrect. (It should be greater that 0.) \nTry to press <Count> button.", "Error", MessageBoxButton.OK, MessageBoxImage.Error); } else { if (ComboBox_Choose.SelectedItem == null) { MessageBox.Show("You should choose a bill from a combobox.", "Error", MessageBoxButton.OK, MessageBoxImage.Error); } else { if (listBox_Category.SelectedItem == null) { MessageBox.Show("You should choose a category.", "Error", MessageBoxButton.OK, MessageBoxImage.Error); } else { _repo.AddHistoryItem(Convert.ToDecimal(textBoxCalculate.Text), ComboBox_Choose.SelectedItem as String, (listBox_Category.SelectedItem as Category).Id); textBoxCalculate.Clear(); ComboBox_Choose.SelectedItem = null; listBox_Category.SelectedItem = null; NavigationService.Navigate(new History()); HistoryInitialised1?.Invoke(); } } } }