void UpdateCollection(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
        {
            switch (e.Action)
            {
            case System.Collections.Specialized.NotifyCollectionChangedAction.Add:
            {
                if (e.NewItems != null && e.NewItems.Count > 0)
                {
                    foreach (firma item in e.NewItems)
                    {
                        if (fk.Where(k => k.id_Kategorie == item.id).Count() == 0)
                        {
                            var k = new SI_SelectedKategorien();
                            k.id_Kategorie = item.id;

                            fk.Add(k);
                            db.AddToSI_SelectedKategorien(k);
                            db.SaveChanges();
                        }
                    }
                }

                break;
            }

            case System.Collections.Specialized.NotifyCollectionChangedAction.Remove:
            {
                if (e.OldItems != null && e.OldItems.Count > 0)
                {
                    foreach (firma item in e.OldItems)
                    {
                        if (fk.Where(k => k.id_Kategorie == item.id).Count() == 1)
                        {
                            var k = fk.Where(ka => ka.id_Kategorie == item.id).SingleOrDefault();
                            fk.Remove(k);
                            db.DeleteObject(k);
                            db.SaveChanges();
                        }
                    }
                }
                break;
            }


            default:
            {
                break;
            }
            }
        }