ACCESS数据库的 城市级联 如何在下拉的控件显示

public void bindCityName()        {            string sql = "select * from TB_Area where parentId=0";            DataSet ds = DBHelp.GetDataSet(sql);            comsheng.DisplayMember = "Name";            comsheng.ValueMember = "Id";            comsheng.DropDownStyle = ComboBoxStyle.DropDownList;            comsheng.DataSource = ds.Tables[0];        }        private void comsheng_SelectedIndexChanged(object sender, EventArgs e)        {                        if (comsheng.SelectedItem != null)            {                comshi.Enabled = true;                string sql = " select*from TB_Area where parentId=comsheng.SelectedItem.value";                DataSet ds=DBHelp.GetDataSet(sql);                comshi.DisplayMember = "Name";                comshi.ValueMember = "Id";                comshi.DropDownStyle = ComboBoxStyle.DropDownList;                comshi.DataSource=ds.Tables[0];            }        }