private bool IsCollectCategoryModified(Billing item) => GetLoadedItem(item)?.CollectCategoryId != item.CollectCategoryId;
private bool IsSameBilling(Billing x, Billing y) => y.BillingInputId.HasValue && x.BillingInputId == y.BillingInputId || !y.BillingInputId.HasValue && x.Id == y.Id;
private bool IsModifiedDueAtModified(Billing item) => GetLoadedItem(item)?.ModifiedDueAt != item.ModifiedDueAt;
private Billing GetLoadedItem(Billing item) => LoadedSource.FirstOrDefault(x => IsSameBilling(x, item));