The QuoteService

In this section of the workshop we will be focusing on unit testing one particular class, the QuoteService which can be found in the Services folder. This class contains 4 main aspects that you should know how to unit test because they appear over and over again in all code-bases.

Those are:

• A returned value
• An IO call, in this case a database call
• An exception thrown
• A method invocation that doesn't return anything

All of these aspects lead to code paths that we need to assert against but the means to do so isn't always obvious.

The GetQuoteAsync method will be what we will be focusing on:

public async Task<ConversionQuote?> GetQuoteAsync(string fromCurrency, string toCurrency, decimal amount)
{
    var sw = Stopwatch.StartNew();
    try
    {
        if (amount <= 0)
        {
            throw new NegativeAmountException();
        }
        
        if (fromCurrency == toCurrency)
        {
            throw new SameCurrencyException(fromCurrency);
        }
    
        var rate = await _ratesRepository.GetRateAsync(fromCurrency, toCurrency);

        if (rate is null)
        {
            return null;
        }

        var quoteAmount = rate.Rate * amount;
    
        return new ConversionQuote
        {
            BaseCurrency = fromCurrency,
            QuoteCurrency = toCurrency,
            BaseAmount = amount,
            QuoteAmount = quoteAmount
        };
    }
    finally
    {
        _logger.LogInformation(
            "Retrieved quote for currencies {FromCurrency}->{ToCurrency} in {ElapsedMilliseconds}ms",
            fromCurrency, toCurrency, sw.ElapsedMilliseconds);
    }
}