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    private void 窗口1ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            //方式1：子窗体会重复打开
            //FrmChild1 child1 = new FrmChild1();
            //child1.MdiParent = this;
            //child1.Show();

            //方式2：子窗体不会重复打开
            bool flag = true;
            foreach (Form item in this.MdiChildren)
            {
                if (item.Name == "FrmChild1")
                {
                    item.Activate();
                    item.WindowState = FormWindowState.Normal;
                    item.Show();
                    flag = false;
                    break;
                }
            }

            if (flag == true)
            {
                FrmChild1 child1 = new FrmChild1();
                child1.Name = "FrmChild1";
                child1.MdiParent = this;
                child1.Show();
            }
        }