WPF Datagrid分组-具有绑定转换器的项目不会动态更新

<DataGrid.GroupStyle>
    ...
 <TextBlock.Text>
        <MultiBinding StringFormat=\"{}{0}, Count: {1:#0}, Executed Qty: {2}\">
            <Binding Path=\"Name\" />
            <Binding Path=\"ItemCount\" />
            <Binding Path=\"Items\" Converter=\"{StaticResource convertToExecutedQty}\" />
        </MultiBinding>
</TextBlock.Text>
.....
</DataGrid.GroupStyle>

public class ConvertToExecutedQty : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (value != null && value is IEnumerable<Object>)
        {
            IEnumerable<TickOrderViewModel> orders = ((IEnumerable<object>) value).GetBaseElements<TickOrderViewModel>();


            if (orders.Count() > 0)
            {
                var BuyQty = ( from n
                              in orders
                               where n.side.ToUpperInvariant() == \"BUY\"
                               select n.executed_qty ).Sum();

                var SellQty = ( from n
                              in orders
                               where n.side.ToUpperInvariant() == \"SELL\"
                               select n.executed_qty ).Sum();

                return BuyQty - SellQty;
            }
        }

        return null;
    }