设置了子父窗体.
父窗体Form1, 上有菜单,打开Form2
Form2 myForm = new Form2();
myForm.MdiParent =this;
myForm.Show();
子窗体Form2,上有菜单,打开Form3
Form3 myForm = new Form3();
myForm.MdiParent =this.MdiParent;;
myForm.Show();
Form3作为Form1的子窗体显示在父窗体内.