//第一次载入
 if (Page.IsPostBack==false)
   {
    CreateDataSet(); 
    BindClass(classList, 0);
   }
关键代码
private DataSet CreateDataSet()
  {//建立数据集
   SqlConnection conn=new SqlConnection();
   conn.ConnectionString=ConfigurationSettings.AppSettings

["SqlConnString"].ToString();
   conn.Open();
   string sql="select * from NewsClass";
   SqlDataAdapter da=new SqlDataAdapter(sql,conn);
   ds=new DataSet();
   da.Fill(ds,"ClassTable");
   da.Dispose();
   conn.Dispose();
   conn.Close();
   classList.Items.Clear();
   classList.Items.Add(new ListItem("作为一级分类","0"));
   return ds;
  }
  private void BindClass(DropDownList List,int parentId)
  {//从根节点开始递归调用显示子树 
   //classList.DataSource=ds.Tables;
   //classList.DataMember="ClassTable";
   //classList.DataBiList();

   DataView dv=new DataView();   
   ListItem tmpList;
   string intId;
   dv.Table=ds.Tables["ClassTable"];
   dv.RowFilter="parentid="+parentId+"";
   foreach(DataRowView drv in dv)
   {
    tmpList=new ListItem();
    string depth="";
    for(int i=0;i<=Int32.Parse(drv["depth"].ToString());i++){
     depth+="--";
    }
    tmpList.Text=depth+drv["classname"].ToString();
    tmpList.Value=drv["classid"].ToString();
    List.Items.Add(tmpList);
    intId=drv["parentid"].ToString();
    if (drv["child"].ToString()=="1")
    {
     BindClass(classList,Int32.Parse(tmpList.Value));
    }    
   }
  }