外部唯一窗体

   private void ShowOrActiveForm<T>(string FormName) where T : Form, new()
        {
            #region
            Form fm;
            if (Application.OpenForms[FormName] != null)
            {
                fm = Application.OpenForms[FormName];
                fm.Activate();
            }
            else
            {
                fm = new T();
                //fm.MdiParent = this;
                fm.Show();
            }
            #endregion
        }