Class to process exchange rates of Czech National Bank (CNB), including historic data and sample console application.

KurzyCnb.cs

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Globalization;
using System.Net;
using System.Threading.Tasks;

namespace Altairis.KurzyCnb {
    class Program {
        static void Main(string[] args) {
            // Get exchange rate
            CnbExchangeRateList erl;
            try {
                Console.Write("Downloading exchange rates from CNB...");
                erl = CnbClient.GetRates();
                Console.WriteLine("OK");
            }
            catch (Exception ex) {
                Console.WriteLine("Failed!");
                Console.WriteLine(ex);
                return;
            }

            // Print rates
            Console.WriteLine();
            Console.WriteLine("    | Country              | Currency   | Amount   | Rate");
            Console.WriteLine(new string('-', 79));
            foreach (var er in erl) {
                Console.WriteLine($"{er.Code} | {er.Country,-20} | {er.Currency,-10} | {er.Amount,8} | {er.Rate,10:N2}");
            }
            Console.WriteLine(new string('-', 79));
            Console.WriteLine();
        }
    }

    public static class CnbClient {

        private const string API_URL_FORMAT = @"https://www.cnb.cz/cs/financni_trhy/devizovy_trh/kurzy_devizoveho_trhu/denni_kurz.txt?date={0:dd\.MM\.yyyy}";

        public async static Task<CnbExchangeRateList> GetRatesAsync(DateTime date) {
            // Download content from CNB
            using (var wc = new WebClient()) {
                wc.Encoding = System.Text.Encoding.UTF8;
                var url = string.Format(API_URL_FORMAT, date);
                var dataString = await wc.DownloadStringTaskAsync(url);
                return CnbExchangeRateList.Parse(dataString);
            }
        }

        public async static Task<CnbExchangeRateList> GetRatesAsync() {
            return await GetRatesAsync(DateTime.Today);
        }

        public static CnbExchangeRateList GetRates(DateTime date) {
            return GetRatesAsync(date).Result;
        }

        public static CnbExchangeRateList GetRates() {
            return GetRatesAsync().Result;
        }

    }

    public class CnbExchangeRateList : ReadOnlyCollection<CnbExchangeRate> {
        public DateTime Date { get; private set; }

        private CnbExchangeRateList(IList<CnbExchangeRate> list, DateTime date) : base(list) {
            this.Date = date;
        }

        internal static CnbExchangeRateList Parse(string s) {
            if (s == null) throw new ArgumentNullException(nameof(s));
            if (string.IsNullOrWhiteSpace(s)) throw new ArgumentException("Value cannot be empty or whitespace only string.", nameof(s));

            // Split data files to lines
            var lines = s.Split('\n');
            if (lines.Length < 3) throw new FormatException($"Invalid data file format - too few lines. Expected at least 3, got {lines.Length}.");

            // Process lines
            var erl = new List<CnbExchangeRate>();
            var date = DateTime.MinValue;
            for (int i = 0; i < lines.Length; i++) {
                if (i == 0) {
                    // First line - date
                    var dateString = lines[0].Substring(0, 10);
                    var result = DateTime.TryParseExact(dateString, @"dd\.MM\.yyyy", CultureInfo.InvariantCulture, DateTimeStyles.None, out date);
                    if (!result) throw new FormatException($"Invalid file format - Error on line 1: {dateString} cannot be parsed as date.");
                }
                else if (i == 1) {
                    // Second line - ignore (headers)
                    continue;
                }
                else {
                    // Other lines - parse
                    CnbExchangeRate er;
                    try {
                        er = CnbExchangeRate.Parse(lines[i]);
                    }
                    catch (Exception e) {
                        throw new FormatException($"Invalid file format - Error on line {i + 1}: \"{lines[i]}\" cannot be parsed as exchange rate.", e);
                    }
                    if (er == null) break;
                    erl.Add(er);
                }
            }
            return new CnbExchangeRateList(erl, date);
        }

    }

    public class CnbExchangeRate {

        internal static CnbExchangeRate Parse(string s) {
            if (string.IsNullOrWhiteSpace(s)) return null;
            var data = s.Split('|');
            if (data.Length != 5) throw new FormatException($"Expected 5 segments, got {data.Length}.");

            return new CnbExchangeRate {
                Country = data[0],
                Currency = data[1],
                Amount = int.Parse(data[2]),
                Code = data[3],
                Rate = decimal.Parse(data[4], CultureInfo.GetCultureInfo("cs-CZ"))
            };
        }

        public string Country { get; set; }

        public string Currency { get; set; }

        public int Amount { get; set; }

        public string Code { get; set; }

        public decimal Rate { get; set; }

        public decimal FromCzk(decimal amount = 1) {
            return amount / this.Rate * this.Amount;
        }

        public decimal ToCzk(decimal amount = 1) {
            return amount * this.Rate / this.Amount;
        }

    }

}