コンボボックス型のセルで、ドロップダウン項目を選択した時に何らかの処理を行うには、PrepareCellForEditイベント内で、エディタのChildプロパティからC1FlexComboBoxを取得し、そのListBox.SelectionChangedイベントを利用することができます。

以下に、簡単な設定コードを記載します。



◎サンプルコード（C#／.NET Framework）

（MainWindow.xaml.cs）
using C1.WPF.FlexGrid;

namespace prj_wpf_FlexGrid
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            List comboList = new List() { "AAAA", "BBBB", "CCCC", "DDDD", "EEEE"};

            // コンボボックスの設定
            c1FlexGrid1.Columns[0].ValueConverter
                = new ColumnValueConverter(comboList, true);
            c1FlexGrid1.PrepareCellForEdit += c1FlexGrid_PrepareCellForEdit;
        }

        private void c1FlexGrid_PrepareCellForEdit(object sender, CellEditEventArgs e)
        {
            if (c1FlexGrid1.Columns[0].Index == e.Column)
            {
                if (e.Editor is Border)
                {
                    var bdr = e.Editor as Border;
                    if (bdr.Child is C1FlexComboBox)
                    {
                        var cmb = bdr.Child as C1FlexComboBox;
                        // 項目選択時の処理
                        cmb.Loaded += (s1, e1) =
                        {
                            cmb.ListBox.SelectionChanged += ListBox_SelectionChanged;
                        };
                    }
                }
            }
        }

        private void ListBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            if (e.RemovedItems.Count  0 || e.AddedItems.Count  0)
                MessageBox.Show("Changed from " + e.RemovedItems[0] + " to "+ e.AddedItems[0]);
        }
    }
}

（ViewModel.cs）
namespace prj_wpf_FlexGrid
{
    public class ViewModel : INotifyPropertyChanged
    {
        //プロパティ
        [field: NonSerialized]
        public event PropertyChangedEventHandler PropertyChanged;
        private void OnPropertyChanged(string propertyName)
        {
            this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

        public ObservableCollection DataList { get; set; }

        public ViewModel()
        {
            DataList = new ObservableCollection();
            DataList.Add(new Data("AAAA"));
        }
    }

    public class Data : INotifyPropertyChanged
    {
        //プロパティ
        [field: NonSerialized]
        public event PropertyChangedEventHandler PropertyChanged;
        private void OnPropertyChanged(string propertyName)
        {
            this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

        public Data(string sampleNo)
        {
            SampleNo = sampleNo;
        }

        private string _SampleNo = "";
        public string SampleNo
        {
            get { return _SampleNo; }
            set
            {
                if (_SampleNo == value) return;
                _SampleNo = value;
                OnPropertyChanged("SampleNo");
            }
        }
    }
}