1.MDI：Multiple document interface
2.当MDI应用程序启动时，首先会显示父窗体，所有的子窗体都在父窗体中打开，每个应用程序只能有一个父窗体，其他子窗体不能移除父窗体的框架区域
3.如果要将某个窗体设置为子窗体，首先必须含有一个父窗体（通过窗体的load事件用IsMdiContainer属性进行语句设定），然后在工程中新建Windows窗体，通过窗体的MdiParent属性来设置

例子代码
这个把Form1设置为父窗体，Form2、Form3、Form4为子窗体

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 WindowsFormsApp11
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            this.IsMdiContainer = true;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Form2 form2 = new Form2();
            form2.MdiParent = this;
            form2.Show();
            //以下form3、form4同理
            Form3 form3 = new Form3();
            form3.MdiParent = this;
            form3.Show();

            Form4 form4 = new Form4();
            form4.MdiParent = this;
            form4.Show();
        }
    }
}

运行结果

运行结果.PNG

当然了，你也可以设置子窗体的排列方式，c#提供MdiLayout枚举类型，可到网上查看，这里用一下TitleHorizontal
代码

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 WindowsFormsApp11
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            this.IsMdiContainer = true;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Form2 form2 = new Form2();
            form2.MdiParent = this;
            form2.Show();
            //以下form3、form4同理
            Form3 form3 = new Form3();
            form3.MdiParent = this;
            form3.Show();

            Form4 form4 = new Form4();
            form4.MdiParent = this;
            form4.Show();

            //设置窗体平铺
            this.LayoutMdi(MdiLayout.TileHorizontal);
        }
    }
}

运行结果

运行结果.PNG