public static string TestGSMCalls()
        {
            StringBuilder result = new StringBuilder();

            GSM testPhone = new GSM("Galaxy S5", "Samsung", 1000M, "Sasho", new Battery("Some model", 390, 21, BatteryType.Li_Ion),
                                    new Display(5.1, 16000000));

            testPhone.AddCall(new Call(DateTime.Now, "+359888888888", 180));
            testPhone.AddCall(new Call(new DateTime(2010, 5, 12), "088888888888", 120));
            testPhone.AddCall(new Call(new DateTime(2012, 6, 7), "08877777777", 60));

            result.AppendLine("Phone calls in history: ");
            result.Append(testPhone.GetCallHistory());

            decimal totalPriceOfCalls = testPhone.CalculateTotalCallPrice(CallPricePerMinute);

            result.AppendFormat("Total price of calls in the history: {0:C}", totalPriceOfCalls);
            result.AppendLine(Environment.NewLine);

            Call longestCall = testPhone.CallHistory.OrderBy(x => x.CallDuration).LastOrDefault();

            testPhone.DeleteCall(longestCall);

            totalPriceOfCalls = testPhone.CalculateTotalCallPrice(CallPricePerMinute);
            result.AppendFormat("Total price of calls in the history after removing the longest call: {0:C}", totalPriceOfCalls);
            result.AppendLine(Environment.NewLine);

            testPhone.ClearCallHistory();
            result.AppendLine("Call history after deletion of all the calls: ");
            try
            {
                result.Append(testPhone.GetCallHistory());
            }
            catch (NullReferenceException exc)
            {
                result.AppendLine(exc.Message);
            }

            return(result.ToString());
        }
        public static string TestGSM()
        {
            StringBuilder result = new StringBuilder();

            GSM testOne = new GSM("Galaxy S5", "Samsung", 1000M, "Sasho", new Battery("Some model", 390, 21, BatteryType.Li_Ion),
                                  new Display(5.1, 16000000));

            GSM testTwo = new GSM("Experia Z3", "Sony", 1000M, "Mustafa", new Battery("Some model", 790, 14, BatteryType.Li_Ion),
                                  new Display(5.2, 16000000));

            GSM testThree = new GSM("Lumia 535", "Microsoft", 1000M, "Asen", new Battery("Some model", 552, 11, BatteryType.Li_Ion),
                                    new Display(5, 16000000));

            GSM testFour = new GSM("Nexus", "Google");

            GSM[] testPhones = { testOne, testTwo, testThree, testFour, GSM.Iphone4s };

            foreach (var phone in testPhones)
            {
                result.AppendLine(phone + Environment.NewLine);
            }

            return(result.ToString());
        }