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

namespace MDI窗体的使用
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            Form2 frmChild1 = null;
            Form3 frmChild2 = null;
        }


        private void 打开子窗体一ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            if (frmChild1 == null)
            {
                frmChild1 = new Form2();
                frmChild1.MdiParent = this;
                frmChild1.Show();
            }
            else
            {
                if (frmChild1.IsDisposed)
                {
                    frmChild1 = new Form2();
                }
                frmChild1.MdiParent = this;
                frmChild1.Show();
                frmChild1.Focus();
            }
        }

        private void 水平排列ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            LayoutMdi(MdiLayout.TileHorizontal);
        }

    }
}

现在父窗体里面设置一个属性、具体啥我也忘了。
不好意思哦、我很久没写cs了。
后面就像这样就行:
  frmChild1 = new Form2();
                frmChild1.MdiParent = this;
                frmChild1.Show();

父窗体里面设置isMdiContainer=true