MonoTouch-如何在清除数据源时立即重新绘制UITableView

在MonoTouch中，当我清除空的UITableView的数据源时，我如何立即重新绘制它？

List<string> ds = new List<string>();
UITableView tbl = new UITableView();
tbl.DataSource = new MyTableViewDataSource(ds);
//And the other parts(datasource class, delegate class etc.) are truely coded when i try to do somewhere at my program, something like
ds.Add("new string");
//it works and table shows new data too, but when i say
ds.Clear();
//it works and clear but table is not redrawn

感谢