[WPF] ComboBox: error multiConverter and Selected Item

Hi,


in my datagrid I insert a TextBlock and ComboBox for edit cell.


When I click in the cell for edit and I select an item of combox:
1) the item selected is not shown in cell of DataGrid.
2) my Convert is called two times.


Why?


XAML












                                           
                                               


                                               








My Converter:


    public class DayNumName : IMultiValueConverter
    {
        public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
        {
            if (values == null || values.Length < 4)
                return null;


            AnalysisPlanItemExt analisysPlanItem = (AnalysisPlanItemExt)values[0];
            List listDayOfWeek = (List)values[1];
            List listDayNumOfMonth = (List)values[2];


            if (analisysPlanItem.Type != null)
            {
                //Day of Weeks
                if (analisysPlanItem.Type.Value.Equals(Definition.SchedTypeEnum.DayOfWeek))
                    return listDayOfWeek;


                if (analisysPlanItem.Type.Value.Equals(Definition.SchedTypeEnum.DayOfMonth))
                    return listDayNumOfMonth;
            }


            return null;
        }


        public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
        {
            throw new System.NotImplementedException();
        }
    }
Thanks!