C# อยากจะทำให้ form mdi เปิดครั้งเดียวและพับ mdi อื่่นให้เล็กลง

private void openForm(string FormName)
        {

            Form activeForm = null;

            foreach (Form childForm in MdiChildren)
            {
                if (childForm.Name.Equals(FormName, StringComparison.OrdinalIgnoreCase))
                {
                    activeForm = childForm;

                }
                else
                    childForm.WindowState = FormWindowState.Minimized;


            }


            if (activeForm == null)
            {
                Form newChild = null;
                if (FormName == "Form1")
                    newChild = new Form1();
                else if (FormName == "Form2")
                    newChild = new Form2();

                if (newChild != null)
                {
                    newChild.Left = 0;
                    newChild.Top = 0;
                    newChild.MdiParent = this;
                    newChild.Show();
                }
            }
            else
            {

                activeForm.Left = 0;
                activeForm.Top = 0;
                activeForm.Activate();
                activeForm.WindowState = FormWindowState.Normal;

            }

        }

private void mDI1ToolStripMenuItem_Click(object sender, EventArgs e)
        {

            openForm("Form1");
        }