添加菜单，IsMdiContainer设为True:

From窗口添加菜单

Form1.cs

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

namespace 多文档界面MDI
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private void 视频ToolStripMenuItem_Click(object sender, EventArgs e)
        {

        }

        private void 编辑ToolStripMenuItem_Click(object sender, EventArgs e)
        {

        }

        private void 窗口ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            //MessageBox.Show("窗口被单击", "操作提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
        }

        private void 子窗口2ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            Form2 Mdichild = new Form2();
            Mdichild.MdiParent = this;
            Mdichild.Show();
        }

        private void 子窗口3ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            
        }

        private void 窗口层叠ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            this.LayoutMdi(MdiLayout.Cascade);
        }

        private void 水平平铺ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            this.LayoutMdi(MdiLayout.TileHorizontal);
        }

        private void 垂直平铺ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            this.LayoutMdi(MdiLayout.TileVertical);
        }
    }
}