前台

后台代码：

先绑定第一级

string SQL = "SELECT CategoryID,CategoryName,ParentID FROM Category"                    DataTable datatable = SqlHelper.ExecuteDataTable(SQL);                 DataRow[] row = datatable.Select("ParentID='0'");                 //添加根目录                 for (int i = 0; i < row.Length; i++)                 {
                        ListItem li = new ListItem(row[i]["CategoryName"].ToString(), row[i]["CategoryID"].ToString());                     ddlOutType.Items.Add(li);                     addOtherDll("", Convert.ToInt32(row[i]["CategoryID"]), datatable, 1, ddlOutType);                 }

递归查找下级

/// 空格        /// 父路径ID        /// 返回的datatable        /// 树形的深度        private void addOtherDll(string Pading, int DirId, DataTable datatable, int deep,DropDownList ddl)        {            DataRow[] rowlist = datatable.Select("ParentID='" + DirId + "'");            foreach (DataRow row in rowlist)            {                string strPading = "";                for (int j = 0; j < deep; j++)                {                    strPading += "　"; //用全角的空格                }                //添加节点                ListItem li = new ListItem(strPading + "|--" + row["CategoryName"].ToString(),row["CategoryID"].ToString());                ddl.Items.Add(li);                //递归调用addOtherDll函数，在函数中把deep加1                addOtherDll(strPading, Convert.ToInt32(row["CategoryID"]), datatable, deep + 1, ddl);            }        }