Unit Testing, Part II, Synchronisation Issues

ComplexBusinessOperations-1.cs

public class ComplexBusinessOperations
{
    private int moneyEarnedSoFar;

    public int MoneyEarnedSoFar
    {
        get { return moneyEarnedSoFar; }
    }

    public void EarnMoney(int investment)
    {
        EarnMoneySubTaks(investment);
        EarnMoneySubTaks(investment);
    }

    private void EarnMoneySubTaks(int investment)
    {
        for (int i = 0; i < investment; i++)
        {
            var result = EarnOneDollar();
            moneyEarnedSoFar += result;
        }
    }

    private int EarnOneDollar()
    {
        Thread.Sleep(50);
        return 1;
    }
}

ComplexBusinessOperationsTest-1.cs

[Test]
public void EarnMoney()
{
    var toTest = new ComplexBusinessOperations();
    toTest.EarnMoney(100);
    Assert.AreEqual(200, toTest.MoneyEarnedSoFar);
}