ForexCalcTest.cs

[Fact]
public void ShouldUseForexRateToCalculatePortfolioPosition()
{
	// set up a stub to return a value that should be used in the calculation
	var forexRateRetriever = new Mock<IRetrieveForexRates>();
	forexRateRetriever
		.Setup(x=>
			x.GetCurrentRateFor(
				It.IsAny<SourceCurrency>(), 
				It.IsAny<DestinationCurrency>())
		.Returns(new ForexRate(ratio:1.13m,...);
        
	// the SUT can have other dependencies that could be stubbed/mocked/faked/dummied
    // appropriately
	var portfolioValueCalculator = CreateSUT(forexRateRetriever.Object,...);
	var portfolioValue = portfolioValueCalculator.Calculate();
	
	// if the forexRateRetriever wasn't being used correctly, this assertion will fail
	portfolioValue.PositionTotal.Should().Be(2000);
}