dropdownlist无限级分组显示

List<column_Model> ddlList = new List<column_Model>();

        /// <summary>
        /// 绑定父栏目
        /// </summary>
        /// <param name="ddlColumn">栏目下拉控件</param>
        public void BindFather(DropDownList ddlColumn)
        {
            ddlColumn.Items.Add(new ListItem("请选择栏目...", "0"));
            ddlList = DAL.column.Select();
            if (ddlList!=null)
            {
                foreach (column_Model item in ddlList)
                {
                    if (item.parentId=="0")
                    {
                        string columnId = item.columnId;
                        string columnName = item.columnName;
                        columnName = "" + columnName;
                        ddlColumn.Items.Add(new ListItem(columnName, columnId));
                        string sonParentId = columnId;
                        string blank ="├";
                        BindNode(sonParentId, blank, ddlColumn);
                    }
                }
                ddlColumn.DataBind();
            }
        }

        /// <summary>
        /// 绑定子栏目
        /// </summary>
        /// <param name="parentid">父栏目ID</param>
        /// <param name="blank">间隔符号</param>
        /// <param name="ddlColumn">栏目下拉控件</param>
        public void BindNode(string parentid, string blank, DropDownList ddlColumn)
        {
            List<column_Model> sonList = getChild(parentid);
            foreach (column_Model item in sonList)
            {
                string columnId = item.columnId;
                string columnName = item.columnName;
                columnName = blank + columnName;
                ddlColumn.Items.Add(new ListItem(columnName, columnId));
                string sonParentId = columnId;
                string blank2 = blank + "─";
                BindNode(sonParentId, blank2, ddlColumn);
            }
        }

        /// <summary>
        /// 根据父栏目ID获取子栏目
        /// </summary>
        /// <param name="parentId">父栏目ID</param>
        /// <returns></returns>
        public List<column_Model> getChild(string parentId)
        {
            List<column_Model> sonList = new List<column_Model>();
            foreach (column_Model item in ddlList)
            {
                if (item.parentId==parentId)
                {
                    sonList.Add(item);
                }
            }
            return sonList;
        }