private IEnumerable<Control> GetAllControls(Control.ControlCollection controls) {
     List<Control> all_controls = new List<Control>();
     foreach (Control control in controls) {
         if (control.GetType() == typeof(GroupBox)) {
             all_controls.AddRange(GetAllControls(control.Controls));
         }
         all_controls.Add(control);
     }
     return all_controls;           
 }