在有父窗体的前提下，判断子窗体是否存在，否则show出

  foreach  (Form form  in  mf.MdiChildren)
             ... {
                if (form.GetType().ToString() == childFormType.ToString())
                ...{
                    form.Focus();
                    return;
                }
            }
            Form formShow  = (Form)Activator.CreateInstance(childFormType,  true );
            formShow.MdiParent  =  mf;
            formShow.Show();
            formShow.Focus();