private int i = 1;
        private void toolStripButton1_Click(object sender, EventArgs e)
        {
            var childForm1 = new ChildForm1 {MdiParent = this, TopLevel = false,Text = "第"+i+"个窗体"};
            childForm1.Show();
            i++;
        }

        private void toolStripButton2_Click(object sender, EventArgs e)
        {
            var childForm2 = new ChildForm2 { MdiParent = this, TopLevel = false, Text = "第" + i + "个窗体" };
            childForm2.Show();
            i++;
        }

        private void toolStripButton3_Click(object sender, EventArgs e)
        {
            //垂直平铺
            this.LayoutMdi(MdiLayout.TileVertical);
        }

        private void toolStripButton4_Click(object sender, EventArgs e)
        {
            //水平平铺
            this.LayoutMdi(MdiLayout.TileHorizontal);
        }

        private void toolStripButton5_Click(object sender, EventArgs e)
        {
            //层叠
            this.LayoutMdi(MdiLayout.Cascade);
        }