<telerik:RadAutoCompleteBox x:Name="RadAutoCompleteBox" FilteringBehavior="{StaticResource EmptyTextFilteringBehavior}" ItemsSource="{Binding Source={local:EnumBindingSource {x:Type model:FlagEnum}}}" SelectedItems="{Binding Entity.FlagEnum, Mode=TwoWay, Converter={StaticResource ListToFlagEnumConverter}}" />

public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
    if (value != null)
    {
        Type type = value.GetType();
        if (typeof(Enum).IsInstanceOfType(value))
        {
            string concatenatedEnum = ((Enum)value).ToString();
            ObservableCollection<Enum> enumList = new ObservableCollection<Enum>();
            foreach (string item in concatenatedEnum.Split(','))
            {
                enumList.Add((Enum)Enum.Parse(type, item));
            }
            return enumList;
        }
    }
    return Binding.DoNothing;
}

public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
    ObservableCollection<Enum> enumList = (ObservableCollection<Enum>)value;
    string enumString = String.Join<object>(",", enumList);
    return Enum.Parse(targetType, enumString);
}