Printset prt;//要显示的窗体
        private void toolStripMenuItem39_Click(object sender, EventArgs e)
        {
            if (prt == null || prt.IsDisposed)
            {
                prt = new Printset();
                prt.MdiParent = this;//父窗体为当前窗体
                prt.Show();
            }
        }