public void Add(ObserverForm of)
 {
     lock (locker)
     {
         _observerForms.Add(of);
     }
 }
 public void Remove(ObserverForm of)
 {
     lock (locker)
     {
         if (_observerForms.Contains(of))
         {
             _observerForms.Remove(of);
         }
     }
 }