下面这个代码是照着书本里写的查了好多次都看不出是什么问题，
提示是这个位置代码错误：ToolStripMenuItem  有四个位置。

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 WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void 加载子窗体 ToolStripMenuItem_Click(object sender,EventArgs e)
        {
            Form2 form2=new Form2();
            form2.MdiParent=this;
            form2.Show();
            Form3 form3=new Form3();
            form3.MdiParent=this;
            form3.Show();
            Form4 form4=new Form4();
            form4.MdiParent=this;
            form4.Show();
        }
        private void 水平平铺 ToolStripMenuItem_Click(object sender,EventArgs e)
        {
            LayoutMdi(MdiLayout.TileHorizontal);
        }
         private void 垂直平铺 ToolStripMenuItem_Click(object sender,EventArgs e)
        {
            LayoutMdi(MdiLayout.TileVertical);
        }
         private void 层叠平铺 ToolStripMenuItem_Click(object sender,EventArgs e)
        {
            LayoutMdi(MdiLayout.Cascade);
        }
    }
}