private async void Calculate() { if ((Until - Since).TotalMilliseconds < 0) { return; } ResponseModel response = await _pk.GetPayedSessionsInPeriod(_userId, Since, Until); IEnumerable <ParkingSession> result = (IEnumerable <ParkingSession>)response.Data; decimal?count = 0; foreach (ParkingSession ps in result) { count += ps.TotalPayment; } CalculatedProfit = count.Value; }