protected void Page_Load(object sender, EventArgs e)
  {
  if (!IsPostBack)
  {
  DataTable datatable = this.GetDataTable();
  DataRow[] row = datatable.Select("ParentID=-1");
  //添加根目录
  for (int i = 0; i <= row.Length-1; i++)
  {
  ListItem li = new ListItem(row[i]["MajName"].ToString());
  //ListItem li2 = new ListItem(row[1]["MajName"].ToString());
  DropDownList1.Items.Add(li);
  //DropDownList1.Items.Add(li2);
  addOtherDll("", Convert.ToInt32(row[i]["MajID"]), datatable, 1);
  }
  }
  }
  /**/
  /// <summary>
  /// 添加其他节点
  /// </summary>
  /// <param name="Pading">空格</param>
  /// <param name="DirId">父路径ID</param>
  /// <param name="datatable">返回的datatable</param>
  /// <param name="deep">树形的深度</param>
  private void addOtherDll(string Pading, int DirId, DataTable datatable, int deep)
  {
  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["MajName"].ToString());
  DropDownList1.Items.Add(li);
  //递归调用addOtherDll函数，在函数中把deep加1
  addOtherDll(strPading, Convert.ToInt32(row["MajID"]), datatable, deep + 1);
  }
  }
  /**/
  /// <summary>
  /// 从数据库中读取数据返回datatable
  /// </summary>
  /// <returns></returns>
  private DataTable GetDataTable()
  {
  SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["ddltest"].ConnectionString);
  SqlDataAdapter adp = new SqlDataAdapter("select * from tblMajor", conn);
  DataSet datset = new DataSet();
  conn.Open();
  adp.Fill(datset, "tblMajor");
  return datset.Tables[0];
  }

dropdownlist无限级联动要怎么取出里面的值的ID.能有人回答一下吗?