C#窗体 MDI窗体写法。

namespace WinFormsApp2
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Click(object sender, EventArgs e)
        {
            MessageBox.Show("提示");
        }

        private void Form1_FormClosing(object sender, FormClosingEventArgs e)
        {
            DialogResult dr = MessageBox.Show("是否关闭", "提示", MessageBoxButtons.YesNo, MessageBoxIcon.Warning);

            if (dr == DialogResult.Yes)
            {
                e.Cancel = false;
            }
            else
            {
                e.Cancel = true;
            }
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            Form2 frm2 = new Form2();
            frm2.Show();
            frm2.MdiParent = this;

            Form3 frm3 = new Form3();
            frm3.Show();
            frm3.MdiParent = this;

        }
    }
}