private void Form3_Load(object sender, EventArgs e)
        {
            this.IsMdiContainer = true;// ,父窗体属性mdicontainer为true
            //学习MDI多窗体
            Form. form11 = new Form();
            form11.Show();
            form11.MdiParent = this;//mdiparent属性设置父窗体

            Form. form12 = new Form();
            form12.Show();
            form12.MdiParent = this;//mdiparent属性设置父窗体
            this.LayoutMdi(MdiLayout.Cascade);//排列父窗体内的子窗体排列方式

                                 
        }