1.DocumentManager要设置自己的MdiParent属性
2.主窗体设置IsMidContainer为True
3.要生成的窗体设置MdiParent为主窗体
4.正常创建窗体，然后就可以显示..Show()
Form formscene = new Form();
formscene.Name = Guid.NewGuid() + "";
formscene.Text = "test";
formscene.Dock = DockStyle.Fill;
formscene.MdiParent = this;
formscene.Show();