using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace WinTest
{
    public partial class Form1 : Form
    {
        DataTable dt;
        public Form1()
        {
            InitializeComponent();
            dt = getdata();
        }

        public DataTable getdata()
        {
            DataTable dt = new DataTable();
            dt.Columns.Add("id");
            dt.Columns.Add("parentID");
            dt.Columns.Add("title");

            Random rand = new Random();

            for (int i = 0; i < 15; i++)
            {
                DataRow dr = dt.NewRow();
                dr[0] = i + 1;
                dr[1] = (15 % (i + 1) == 0) ? "" : rand.Next(1, 10).ToString();
                dr[2] = "name_" + (i + 1).ToString();
                dt.Rows.Add(dr);
            }

            return dt;
        } 

        private void button2_Click(object sender, EventArgs e)
        {
            DataRow[] drows = dt.Select("parentID=''");
            if (drows.Length > 0)
            {
                foreach (DataRow item in drows)
                {
                    ToolStripMenuItem toolitem = new ToolStripMenuItem(item["title"].ToString());
                    DataRow[] children = dt.Select(string.Format("parentID='{0}'", item["id"].ToString()));
                    if (children.Length > 0)
                    {
                        bind(toolitem, item);
                    }
                    menuStrip2.Items.Add(toolitem);
                }
            }            
        }

        private void bind(ToolStripMenuItem item,DataRow dr)
        {
            DataRow[] rows =  dt.Select(string.Format("parentID='{0}'",dr["id"].ToString()));
            if (rows.Length > 0)
            {
                foreach (DataRow drv in rows)
                {
                    bind(item,drv);                 
                }
            } 
            else
            {
                item.DropDownItems.Add(dr["title"].ToString());
            }
        }
    }
}

------解决方案--------------------
上面的代码在构造DataTable的时候有问题，不好意思。请看下面的代码。这个是调试通过的。

C# code
 private void button2_Click(object sender, EventArgs e)
        {
            InitTree();
        }
        public void InitTree()
        {
            DataView dv = this.GetData();
            dv.RowFilter = "ParentID=0";
            foreach (DataRowView drv in dv)
            {
                ToolStripMenuItem node = new ToolStripMenuItem();
                node.Text = drv["cname"].ToString();
                this.menuStrip1.Items.Add(node);
                BuildTree(dv, node,drv["cid"].ToString());
            }
        }
        private void BuildTree(DataView dv, ToolStripMenuItem pnode,string parentID)
        {
            dv.R