public ExchangeController(IMemoryCache memorycache, ILogger <ExchangeController> logger)
        {
            _exchange_service = new ExchangeService();
            _cache            = memorycache;
            _logger           = logger;
            _exchange_service.GetExchangeInfo();
            List <ExchangeInfo> list = _exchange_service.GetList();

            foreach (var item in list)
            {
                _cache.Set(item.Currency, item, TimeSpan.FromDays(1));
            }
        }