C#中的ActiveMdiChild事件,如何判断子窗体是关闭还是激活?

有一个父窗体MainForm,两个子窗体SetingRoomForm,SetingRoomTypeForm,如何在关闭SetingRoomForm时通知MainForm将有关子窗体SetingRoomForm的菜单删除
  //激活或关闭子窗体事件
        private void MainForm_MdiChildActivate(object sender, EventArgs e)
        {
            if (this.ActiveMdiChild is SetingRoomForm)
            {
                SetingRoomForm srf = this.ActiveMdiChild as SetingRoomForm;
                //如果srf被释放，移除窗口菜单中的相应菜单
                if (srf.IsDisposed)
                {
                    foreach (ToolStripMenuItem tsmi in 窗口WToolStripMenuItem.DropDownItems)
                    {
                        if (tsmi.Text.Equals("客房信息管理"))
                        {
                            窗口WToolStripMenuItem.DropDownItems.Remove(tsmi);
                            if (窗口WToolStripMenuItem.DropDownItems.Count == 3)
                            {
                                窗口WToolStripMenuItem.DropDownItems.RemoveAt(2);
                            }
                            客房信息管理ToolStripMenuItem.Enabled = true;
                        }
                    }
                }
                else
                {
                    srf.FormRefresh();
                }
            }
            else if(this.ActivateMdiChild is SetingRoomTypeForm)
            {
                SetingRoomTypeForm srtf = this.ActiveMdiChild as SetingRoomTypeForm;
                if (srtf.IsDisposed)
                {
                    foreach (ToolStripMenuItem tsmi in 窗口WToolStripMenuItem.DropDownItems)
                    {
                        if (tsmi.Text.Equals("客房类型管理"))
                        {
                            窗口WToolStripMenuItem.DropDownItems.Remove(tsmi);
                            if (窗口WToolStripMenuItem.DropDownItems.Count == 3)
                            {
                                窗口WToolStripMenuItem.DropDownItems.RemoveAt(2);
                            }
                            客房类型管理ToolStripMenuItem.Enabled = true;
                        }
                    }
                }
                else
                {
                    srtf.FormRefresh();
                }
            }
        }
请大家帮忙说下,用什么方法可以做出来,谢谢