public void GetNetSalarySouldCallTaxCalculatorFactory()
        {
            Salary grossSalary = SalaryProvider.Get();

            ITaxCalculator taxCalculator = Mock.Of <ITaxCalculator>();

            Mock.Get(_taxCalculatorFactory)
            .Setup(tcf => tcf.GetForCurrency(grossSalary.Currency))
            .Returns(taxCalculator);

            _service.GetNetSalary(grossSalary);

            Mock.Get(_taxCalculatorFactory)
            .Verify(tcf => tcf.GetForCurrency(grossSalary.Currency), Times.Once);
        }