C# 中 tree 结构的显示

//访问数据库
        public DataTable gettable(String sql) 
        {
             SqlConnection sqlcon =null;
             try
             {
                 //得到连接字符串
                 String constr = "Data Source=.;Initial Catalog=Test;Persist        Security    Info=True;User ID=sa;Password=sasa";
                 //得到连接对象
                 sqlcon = new SqlConnection(constr);
                 //打开连接
                 sqlcon.Open();
                 //获得执行对象
                 SqlCommand comand = new SqlCommand(sql, sqlcon);
                 SqlDataAdapter data = new SqlDataAdapter(comand);
                 DataSet set = new DataSet();
                 data.Fill(set);
                 return set.Tables[0];
             }
             catch (Exception ex)
             { 
                 throw ex ;
             }
             finally 
             {
                    //关闭连接
                    sqlcon.Close();
             }

DataTable table = null;
          //加载
        private void Form1_Load(object sender, EventArgs e)
        {
            //查询所有数据
            string sql = "select *from test ";
            table = gettable(sql);
            //从datea中帅选 一级菜单
            DataRow[] arr = table.Select("pid =0 "); //父id
            foreach(DataRow row in arr)
            {
                TreeNode node = new TreeNode(row["name"].ToString());
                node.Tag =row["id"].ToString();
                addnode(node); //是否有子节点
                this.treeView1.Nodes.Add((node));
            }
          }

     //递归 显示子节点
          public void addnode(TreeNode node) 
        {
            string id = node.Tag.ToString();
            DataRow[] arr = table.Select("pid = "+ id ); //子节点
            foreach (DataRow row in arr)
            {
                TreeNode nodenode = new TreeNode(row["name"].ToString());
                nodenode.Tag = row["id"].ToString();
                node.Nodes.Add(nodenode);
                addnode(nodenode) ;        // 子节点是否还有子节点
                //递归自己调用自己必须有退出的条件否则 死循环      

             }
         }