Unofficial https://www.currencyconverterapi.com/ C# Library
Requests goes to https://free.currencyconverterapi.com/api/v6/ base url without api key;
var converter = new Converter();
Requests goes to https://api.currencyconverterapi.com/api/v6/ base url with api key;
var converter = new Converter("[YOUR_API_KEY]");
var result = converter.Convert(1, CurrencyType.USD, CurrencyType.EUR);
var history = converter.GetHistory(CurrencyType.USD, CurrencyType.EUR, "2018-08-01");
var historyRange = converter.GetHistoryRange(CurrencyType.USD, CurrencyType.EUR, "2018-08-01", "2018-08-06");
var currencies = converter.GetAllCurrencies();
var countries = converter.GetAllCountries();
var basic = await converter.ConvertAsync(1, CurrencyType.USD, CurrencyType.EUR);
var history = await converter.GetHistoryAsync(CurrencyType.USD, CurrencyType.EUR, "2018-08-01");
var historyRange = await converter.GetHistoryRangeAsync(CurrencyType.USD, CurrencyType.EUR, "2018-08-01", "2018-08-06");
var currencies = await converter.GetAllCurrenciesAsync();
var countries = await converter.GetAllCountriesAsync();