public Offer AssignOffer(OfferType offerType, IOfferValueCalculator valueCalculator)
        {
            DateTime dateExpiring = offerType.CalculateExpirationDate();
            int value = valueCalculator.CalculateValue(this, offerType);


            var offer = new Offer(this, offerType, dateExpiring, value);


            _assignedOffers.Add(offer);


            NumberOfActiveOffers++;


            return offer;
        }