First encounters w ith unit testing in C#

Printer QR-Code in C# First encounters w ith unit testing

In chapter 1, we introduced a small application that ll stay with you through your journey with CI. It s your friend the leasing/credit calculator. It can calculate the credit rate according to several input variables such as contract duration and interest. But before we dive into some mathematical details of finance, let s change the calculator a little by flattening the structure. For better clarity, you ll keep all the projects in your solution at one level. If you like the structure with external SVN references, feel free to keep the project that way; but here you ll modify it. From now on, you ll have one solution named CiDotNet with some projects inside, including the calculation core in a project named CiDotNet.Calc (it contains basically what the Framework external SVN reference repository had). The Windows calculator is in the project CiDotNet.WinCalc, the web calculator is in CiDotNet.WebCalc, and the Silverlight calculator is in CiDotNet.SilverlightCalc. The sources provided with this book include a ready-to-use project. Let s start with the calculation core and its mathematical details. This information isn t necessary from the CI point of view, but it s important to fully understand the unit tests that will follow. If you re a unit testing specialist, please feel free to skip the next section.

It s time to add code to the project so you have something to unit test. Open the class library project CiDotNet.Calc and add a new class named FinanceHelper, as shown next. Listing 6.1

using System; namespace Core.Math { public class Finance { public enum Mode { BeginMode = 1, EndMode = 0 } private static double CalculateSPPV(double compoundPeriods, double periodicInterestRate) { return System.Math.Pow(1.0 + (periodicInterestRate / 100), -compoundPeriods); } private double CalculateSPFV(double compoundPeriods, double periodicInterestRate) { return System.Math.Pow(1 + (periodicInterestRate / 100), compoundPeriods); } private double CalculateUSPV(double compoundPeriods, double periodicInterestRate) { double uspv = (1 - CalculateSPPV(compoundPeriods, periodicInterestRate)) / (periodicInterestRate / 100); return uspv; } private double CalculateUSFV(double compoundPeriods, double periodicInterestRate) { double usfv = (CalculateSPFV(compoundPeriods, periodicInterestRate) - 1) / (periodicInterestRate / 100); return usfv; } private static double GetCompoundPeriods(int periods, int ppy) { return (double)((ppy * periods) / 12); } private static double GetPeriodicInterestRate( double interestRate, int ppy) {

return (interestRate / ((double)ppy)); } public static double CalculateRate(int periods, int ppy, double interest, double presentValue, double finalValue, Mode mode) { int m = (int)mode; double compoundPeriods = GetCompoundPeriods(periods, ppy); double periodicInterestRate = GetPeriodicInterestRate(interest, ppy); return -((finalValue * CalculateSPPV(compoundPeriods, periodicInterestRate) - presentValue) / ((1.0 + ((periodicInterestRate * m) / 100)) * CalculateUSPV(compoundPeriods, periodicInterestRate))); } } }

This code seems to include lots of cryptic methods and values, but it s much easier to understand than you can tell at first glance. Single Payment Present Value (SPPV) is the present value of money received in the future at a given interest rate. Single Payment Future Value (SPFV) is the future value of money paid in the future at a given interest rate. Uniform Series Present Value (USPV) is the payment required each period to achieve the future value. And Uniform Series Future Value (USFV) is the future value of a uniform payment. All of these values are used in the last and most important calculation: the monthly payment, with the public method CalculateRate(). It takes as parameters all the necessary data to make the calculation, as shown in table 6.1.

Table 6.1 Parameters to the CalculateRate() method Parameter Description Number of periods you want to carry the burden of the loan Periods per year for example, 12 for monthly payments How much the bank charges you (the interest rate) How much money you need right now How much money you need at the end of the loan Whether the bank calculates interest income at the beginning of the calculation period or at the end