// Load-And-Store-Methods

        private void _LoadAllGridData(String month)

        {
            if (!_firstTimeMonthChanged)
            {
                StoreAllCollections();
            }

            try
            {
                MyCollectionA = new ObservableCollection <ExcelA>(_manager.LoadCollectionExcelA(month));
                MyCollectionB = new ObservableCollection <ExcelB>(_manager.LoadCollectionExcelB());
                MyCollectionC = new ObservableCollection <ExcelC>(_manager.LoadCollectionExcelC(month));
            }
            catch (System.IO.IOException e)
            {
                var x = MessageBoxService.ShowMessage("Close all the opened excel files and restart the app!", "Error", MessageButton.OK, MessageIcon.Information);
                if (x.Equals(MessageResult.OK))
                {
                    Environment.Exit(0);
                }
                return;
            }

            MyCollectionBMains    = new ObservableCollection <String>(_manager.LoadCollectionBMains(MyCollectionB.ToList()));
            MyCollectionBForTable = new ObservableCollection <ExcelA>();
        }
        public override ValidationResult Validate(object value, CultureInfo cultureInfo)
        {
            ExcelManager  _manager = new ExcelManager();
            List <ExcelB> listB    = _manager.LoadCollectionExcelB();

            ExcelA entry = (value as BindingGroup).Items[0] as ExcelA;

            if (string.IsNullOrEmpty(entry.Val1))
            {
                return(new ValidationResult(false, "Value 1 must not be empty!"));
            }

            bool contains = false;

            foreach (var item in listB)
            {
                if (item.Val3Main.Equals(entry.Val3))
                {
                    contains = true;
                }
            }

            if (contains)
            {
                return(ValidationResult.ValidResult);
            }
            else
            {
                return(new ValidationResult(false, "Value 3 must exist in ExcelB!"));
            }
        }