using System;
namespace CodeSamples.Ch03_UnitTesting.Listing8
{
  public class FinancialFunctions
  {
    /// <summary>
    /// P = (Pv*R) / [1 - (1 + R)^(-n)]
    /// where
    ///     Pv  = Present Value (beginning
    ///           value or amount of loan)
    ///     APR = Annual Percentage Rate
    ///           (one year time period)
    ///     R   = Periodic Interest Rate =
    ///           APR/ # of interest periods per year
    ///     P   = Monthly Payment
    ///     n   = # of interest periods for overall
    ///           time period (i.e., interest
    ///           periods per year * number of years)
    /// </summary>
    /// <param name="pLoanAmount">Original amount of the loan</param>
    /// <param name="pYearlyInterestRate">Yearly interest rate</param>
    /// <param name="pNumMonthlyPayments">Num of mthly payments</param>
    /// <returns></returns>
    public double CalcLoanPayment(double pLoanAmount,
      double pYearlyInterestRate, int pNumMonthlyPayments)
    {
      //start code change *******************************
      if (pYearlyInterestRate < 0)
        throw new ArgumentException(
          "pYearlyInterestRate cannot be negative");
      //end code change *******************************
      var mthlyInterestRate = pYearlyInterestRate / 1200;
      var numerator = pLoanAmount * mthlyInterestRate;
      var denominator = (1 - Math.Pow(1 + mthlyInterestRate,
            -pNumMonthlyPayments));
      var mthlyPayment = Math.Round(numerator / denominator, 2);
      return mthlyPayment;
    }
  }
}

using System;
using CodeSamples.Ch03_UnitTesting.Listing8;
using Microsoft.VisualStudio.TestTools.UnitTesting;
namespace CodeSamples.Ch03_UnitTesting.Listing9
{
  [TestClass]
  public class FinancialFunctionsUnitTest
  {
    [TestMethod]
    [ExpectedException(typeof(ArgumentException))]
    public void CalcLoanPayment_Loan1000IntNeg10Pmt12_Throws()
    {
      //Arrange
      var ex = new FinancialFunctions();
      //Act
      ex.CalcLoanPayment(1000, -10, 12);
    }
    [TestMethod]
    public void CalcLoanPayment_Loan1000Int10Pmt360_ReturnsPmt()
    {
      //Arrange
      var ex = new FinancialFunctions();
      //Act
      var pmt = ex.CalcLoanPayment(1000, 10, 360);
      //Assert
      Assert.AreEqual(8.78, pmt);
    }
    [TestMethod]
    public void CalcLoanPayment_Loan1000Int5Pmt360_ReturnsPmt()
    {
      //Arrange
      var ex = new FinancialFunctions();
      //Act
      var pmt = ex.CalcLoanPayment(1000, 15, 360);
      //Assert
      Assert.AreEqual(12.64, pmt);
    }
    [TestMethod]
    public void CalcLoanPayment_Loan1000Int10Pmt12_ReturnsPmt()
    {
      //Arrange
      var ex = new FinancialFunctions();
      //Act
      var pmt = ex.CalcLoanPayment(1000, 10, 12);
      //Assert
      Assert.AreEqual(87.92, pmt);
    }
  }
}