public void ExchangeShouldCalculateCorrectly()
{
// Arrange
var repository = new Mock <IExchangeRateRepository>();
repository.Setup(x => x.GetExchangeRate(It.IsAny <string>(), It.IsAny <string>())).ReturnsAsync(0.8m);
var sut = new ExchangeRateService(repository.Object);
// Act
var result = sut.Exchange("AUD", "USD", 100);
// Assert
Assert.AreEqual(result, 80m);
}