多个打开的windows窗体和C#

using System;
using System.Windows.Forms;

public class MainForm : Form {
    private Button openButton;

    public MainForm() {
        openButton = new Button { Text = "Open New Form", Dock = DockStyle.Top };
        openButton.Click += OpenButton_Click;
        this.Controls.Add(openButton);
    }

    private void OpenButton_Click(object sender, EventArgs e) {
        Form newForm = new Form();
        newForm.Text = "New Form";
        newForm.FormClosed += NewForm_FormClosed;
        newForm.Show();
    }

    private void NewForm_FormClosed(object sender, FormClosedEventArgs e) {
        MessageBox.Show("Form closed!");
    }

    [STAThread]
    static void Main() {
        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);
        Application.Run(new MainForm());
    }
}