如何在一个父窗体中只打开一个窗口

        try
            {

                bool isOpen = false;
                foreach (Form frm in this.ParentForm.MdiChildren)
                {
                    //if (this.ActiveMdiChild == frm)//如果窗口為當前活動的子窗口才處理                        
                    if (frm.Name == "frmQAReport")
                    {
                        isOpen = true;
                        ShareForm mf = frm as ShareForm;
                        mf.RefreshQAReport();
                        frm.Activate();
                    }

                }

                if (isOpen == false)
                {
                    Form ofrmQAReport = new frmQAReport();
                    ofrmQAReport.MdiParent = this.ParentForm;
                    ofrmQAReport.WindowState = FormWindowState.Maximized;
                    ofrmQAReport.Show();
                    ofrmQAReport.Activate();
                }
            }
            catch (Exception Err)
            {
                MessageBox.Show(Err.Message);
            }