public void Init()
        {
            //_id = 0 => Add datum(s) to a new prestatie
            //_id != 0 => Add datum(s) to an existing prestatie. The id of this prestatie will be saved in this variable
            _id = 0;
            NewDates = new ObservableCollection<Datum>();
            AddDatum = new Datum {Date = DateTime.Now};

            if (ViewSource != null)
                ViewSource.Source = NewDates;
            else
            {
                ViewSource = new CollectionViewSource {Source = NewDates};
                //DataGrid ordenen op datum (oudste vanboven)
                ViewSource.SortDescriptions.Add(new SortDescription("Date", ListSortDirection.Ascending));
            }
        }
        public void Add()
        {
            var duplicate = ViewSource.View.Cast<Datum>().FirstOrDefault(d => d.Date.Date == AddDatum.Date.Date);
            if (duplicate == null)
            {
                //Add new date to a new prestatie
                if (_id == 0)
                    NewDates.Add(AddDatum);
                //Add new date to an existing prestatie
                else
                {
                    AddDatum.PrestatieId = _id;
                    _ctx.Datums.Add(AddDatum);
                }

                AddDatum = new Datum { Date = DateTime.Now };
            }
            else
            {
                MessageBoxService messageBoxService = new MessageBoxService();
                messageBoxService.ShowMessageBox("Deze datum is al toegevoegd!");
            }
        }