对于ShowDialog,see MSDN：

Because a form displayed as a dialog Box is not closed,you must call the Dispose method of the form when the form is no longer needed by your application.

为了证明这一点：

Form main = new Form();
Form test = new Form();
test.Text = "Close me";
test.Disposed += delegate {
    main.Text = "Second form was disposed";
};
main.Shown += delegate {
    test.Show();
};
Application.Run(main);