DropDownList中如何添加2级下拉菜单

 if (!IsPostBack)
            {
              List<Model.categroy>  list= new DAL.categroy().GetListArray("");
 //取出所有数据放入泛型list中 效率更高
              var q = list.Where(x => x.pid == 0);
//linq语句  x代表list中的每一项 如果pid为0则取出放入q中
              foreach (Model.categroy item in q)
              {
                  DropDownList1.Items.Add(new ListItem(item.caname,item.id.ToString()));
//父级菜单
                  var q2 = list.Where(y => y.pid == item.id);

                  foreach (var item2 in q2)
                  {
                      DropDownList1.Items.Add(new ListItem("......" + item2.caname, item2.id.ToString()));
//二级菜单
                      
                  }
              }
            }