ExchangeRateClient.cs

public class ExchangeRateClient : IExchangeRateClient
{
    private readonly HttpClient _httpClient;
    private readonly IMemoryCache _memoryCache;

    public ExchangeRateClient(HttpClient httpClient, IMemoryCache memoryCache)
    {
        _httpClient = httpClient;
        _memoryCache = memoryCache;
    }

    private string Key(DateOnly date)
    {
        return $"Rate:{date.ToString("yyyy-MM-dd")}";
    }

    public async Task<decimal> Rate(DateOnly date, Currency baseCurrency, Currency toCurrency)
    {
        return await _memoryCache.GetOrCreateAsync(Key(date), async _ =>
        {
            var result = await _httpClient.GetFromJsonAsync<JsonResponse>(
                $"https://data.fixer.io/api/{date.ToString("yyyy-MM-dd")}?base={baseCurrency}&symbols={toCurrency}");
            if (result == null)
            {
                throw new InvalidOperationException("Not a valid JSON response.");
            }

            return result.Rates.Single(x => x.Key == toCurrency.ToString()).Value;
        });
    }
    
    private class JsonResponse
    {
        public Dictionary<string, decimal> Rates { get; set; }
    }
}