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

                                fk.Add(k);
                                db.AddToTermin_TeilnehmerExtern(k);
                                //     db.SaveChanges();
                            }
                        }
                    }

                    break;
                }

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


                default:
                {
                    break;
                }
                }
            }
            catch (Exception ex)
            {
                CommonTools.Tools.ErrorMethods.ShowErrorMessage(ex);
            }
        }