关键见加大加粗字体部分
窗体：两个文本框textBox1、textBox2;一组三级下拉菜单menuItem1、menuItem2、menuItem3。Text属性分别为1，2，3。
源代码如下
using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;

namespace 数组2
{
    /// <summary>
    /// Form1 的摘要说明。
    /// </summary>
    public class Form1 : System.Windows.Forms.Form
    {
        private System.Windows.Forms.MainMenu mainMenu1;
        private System.Windows.Forms.MenuItem menuItem1;
        private System.Windows.Forms.MenuItem menuItem2;
        private System.Windows.Forms.MenuItem menuItem3;
        private System.Windows.Forms.TextBox textBox1;
        private System.Windows.Forms.TextBox textBox2;
        /// <summary>
        /// 必需的设计器变量。
        /// </summary>
        private System.ComponentModel.Container components = null;

        public Form1()
        {
            //
            // Windows 窗体设计器支持所必需的
            //
            InitializeComponent();

            //
            // TODO: 在 InitializeComponent 调用后添加任何构造函数代码
            //
        }

        /// <summary>
        /// 清理所有正在使用的资源。
        /// </summary>
        protected override void Dispose( bool disposing )
        {
            if( disposing )
            {
                if (components != null) 
                {
                    components.Dispose();
                }
            }
            base.Dispose( disposing );
        }

        #region Windows 窗体设计器生成的代码
        /// <summary>
        /// 设计器支持所需的方法 - 不要使用代码编辑器修改
        /// 此方法的内容。
        /// </summary>
        private void InitializeComponent()
        {
            this.mainMenu1 = new System.Windows.Forms.MainMenu();
            this.menuItem1 = new System.Windows.Forms.MenuItem();
            this.menuItem2 = new System.Windows.Forms.MenuItem();
            this.menuItem3 = new System.Windows.Forms.MenuItem();
            this.textBox1 = new System.Windows.Forms.TextBox();
            this.textBox2 = new System.Windows.Forms.TextBox();
            this.SuspendLayout();
            // 
            // mainMenu1
            // 
            this.mainMenu1.MenuItems.AddRange(new System.Windows.Forms.MenuItem[] {
                                                                                      this.menuItem1});
            // 
            // menuItem1
            // 
            this.menuItem1.Index = 0;
            this.menuItem1.MenuItems.AddRange(new System.Windows.Forms.MenuItem[] {
                                                                                      this.menuItem2});
            this.menuItem1.Text = "1";
            // 
            // menuItem2
            // 
            this.menuItem2.Index = 0;
            this.menuItem2.MenuItems.AddRange(new System.Windows.Forms.MenuItem[] {
                                                                                      this.menuItem3});
            this.menuItem2.Text = "2";
            // 
            // menuItem3
            // 
            this.menuItem3.Index = 0;
            this.menuItem3.Text = "3";
            this.menuItem3.Click += new System.EventHandler(this.menuItem3_Click);
            // 
            // textBox1
            // 
            this.textBox1.Location = new System.Drawing.Point(48, 104);
            this.textBox1.Name = "textBox1";
            this.textBox1.Size = new System.Drawing.Size(168, 21);
            this.textBox1.TabIndex = 0;
            this.textBox1.Text = "textBox1";
            // 
            // textBox2
            // 
            this.textBox2.Location = new System.Drawing.Point(48, 152);
            this.textBox2.Name = "textBox2";
            this.textBox2.Size = new System.Drawing.Size(168, 21);
            this.textBox2.TabIndex = 1;
            this.textBox2.Text = "textBox2";
            // 
            // Form1
            // 
            this.AutoScaleBaseSize = new System.Drawing.Size(6, 14);
            this.ClientSize = new System.Drawing.Size(292, 273);
            this.Controls.Add(this.textBox2);
            this.Controls.Add(this.textBox1);
            this.Menu = this.mainMenu1;
            this.Name = "Form1";
            this.Text = "Form1";
            this.ResumeLayout(false);

        }
        #endregion

        /// <summary>
        /// 应用程序的主入口点。
        /// </summary>
        [STAThread]
        static void Main() 
        {
            Application.Run(new Form1());
            
        }

        private void menuItem3_Click(object sender, System.EventArgs e)
        {
            if (menuItem1!=null)
                textBox1.Text="经过名为1的第一级菜单";
            if (menuItem2!=null)
               textBox2.Text="经过名为2的第二级菜单";
        }
    }
}