Tests Async

using Newtonsoft.Json;
using System.Collections.Concurrent;
using RestAPIStoreApp.Management.Database;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.IO;
using System.Linq;
using System.Net.Http;
using System.Runtime.Serialization.Json;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace Tests.Service
{
    class ServiceManager : IDisposable
    {
        private const int PAGINATION = 3;
        private const string API_KEY = "..."; //use your custom API key here
        private const string BASE_URL = "http://api.timezonedb.com/?zone={0}&format=json&key={1}";

        private DataContractJsonSerializer _serializer = new DataContractJsonSerializer(typeof(TimeModel));

        private List<TimeModel> _mainBuffer = new List<TimeModel>(PAGINATION);
        private List<TimeModel> _secondaryBuffer;

        internal delegate void OnDownloadsFinishedHandler(object sender, EventArgs e);
        internal event OnDownloadsFinishedHandler OnDownloadsFinished;

        private List<HttpClient> clients;

        private ServiceManager()
        {
            clients = new List<HttpClient>();
            for (int i = 0; i < TimeZonesCount; i++)
            {
                clients.Add(new HttpClient());
            }
        }
        void IDisposable.Dispose()
        {
            foreach (var client in clients)
                client.Dispose();
        }

        private void HandleBufferFull(List<TimeModel> localBuffer)
        {
            DoSomethingWithThisBuffer(localBuffer); //save to DB, display to UI, whatever... you're supposed to have an array of PAGINATION size here
            localBuffer.Clear();
        }
        private static Mutex mutex = new Mutex();

        internal async Task StartDownloading()
        {
            IEnumerable<Task> tasks =Enumerable.Range(0, TimeZonesCount - 1).Select(index =>
              Task.Run(async () =>
              {
                  var lastBatch = false;
                  if (TimeZonesCount - index - 1 < PAGINATION)
                  {
                      lastBatch = true;
                      _secondaryBuffer = new List<TimeModel>(TimeZonesCount % PAGINATION);
                  }

                  List<TimeModel> localBuffer;
                  bool hasMutex = false;
                  if (lastBatch)
                  {
                      localBuffer = _secondaryBuffer;
                  }
                  else
                  {
                      localBuffer = _mainBuffer;
                  }
                  try
                  {

                      var query = string.Format(BASE_URL, zones[index], API_KEY);

                      var response = await clients[index].GetAsync(query);
                      var json = await response.Content.ReadAsStringAsync();
                      TimeModel model = Deserialize(json);

                      hasMutex = mutex.WaitOne();
                      if (!hasMutex)
                          throw new Exception("MutexException");

                      localBuffer.Add(model);
                  }
                  catch (ObjectDisposedException ode)
                  {
                      throw;
                  }
                  catch (Exception e)
                  {
                      while (!hasMutex)
                          hasMutex = mutex.WaitOne();
                      TimeModel model = new TimeModel();
                      model.ZoneName = zones[index];
                      model.IsError = true;
                      localBuffer.Add(model);
                  }
                  finally
                  {

                      try
                      {
                          if (localBuffer.Count == localBuffer.Capacity && hasMutex)
                          {
                              HandleBufferFull(localBuffer);
                          }
                          if (hasMutex)
                          {
                              mutex.ReleaseMutex();
                          }
                      }
                      catch (Exception finallyException)
                      {

                          throw;
                      }
                  }
                  return 0;
              }));

            try
            {
                await Task.WhenAll(tasks);

                /*
                foreach(var task in tasks)
                {
                    await task;
                }
                */

                /*
                foreach(var task in tasks)
                {
                    if(task.Status != TaskStatus.RanToCompletion)
                    {
                        await task;
                    }
                }
                */
                OnDownloadsFinished(this, new EventArgs());
            }
            catch (Exception e)
            {
                throw;
            }
        }
        internal TimeModel Deserialize(string json)
        {
            var model = JsonConvert.DeserializeObject<TimeModel>(json);
            return model;
        }

        internal int TimeZonesCount
        {
            get
            {
                return zones.Length;
            }
        }

        #region Singleton Impl
        private static ServiceManager _instance;
        internal static ServiceManager Instance
        {
            get
            {
                if (_instance == null)
                    _instance = new ServiceManager();

                return _instance;
            }
        }
        #endregion

        #region Time Zones
        private readonly string[] zones =
        {
            "Europe/Andorra",
            "Asia/Dubai",
            "Asia/Kabul",
            "America/Antigua",
            "America/Anguilla",
            "Europe/Tirane",
            "Asia/Yerevan",
            "Africa/Luanda",
            "Antarctica/McMurdo",
            "Antarctica/South_Pole",
            "Antarctica/Rothera",
            "Antarctica/Palmer",
            "Antarctica/Mawson",
            "Antarctica/Davis",
            "Antarctica/Casey",
            "Antarctica/Vostok",
            "Antarctica/DumontDUrville",
            "Antarctica/Syowa",
            "Antarctica/Macquarie",
            "America/Argentina/Catamarca",
            "America/Argentina/La_Rioja",
            "America/Argentina/San_Juan",
            "America/Argentina/Mendoza",
            "America/Argentina/San_Luis",
            "America/Argentina/Rio_Gallegos",
            "America/Argentina/Ushuaia",
            "America/Argentina/Buenos_Aires",
            "America/Argentina/Cordoba",
            "America/Argentina/Salta",
            "America/Argentina/Jujuy",
            "America/Argentina/Tucuman",
            "Pacific/Pago_Pago",
            "Europe/Vienna",
            "Australia/Lord_Howe",
            "Australia/Hobart",
            "Australia/Currie",
            "Australia/Melbourne",
            "Australia/Sydney",
            "Australia/Broken_Hill",
            "Australia/Brisbane",
            "Australia/Lindeman",
            "Australia/Adelaide",
            "Australia/Darwin",
            "Australia/Perth",
            "Australia/Eucla",
            "America/Aruba",
            "Europe/Mariehamn",
            "Asia/Baku",
            "Europe/Sarajevo",
            "America/Barbados",
            "Asia/Dhaka",
            "Europe/Brussels",
            "Africa/Ouagadougou",
            "Europe/Sofia",
            "Asia/Bahrain",
            "Africa/Bujumbura",
            "Africa/Porto-Novo",
            "America/St_Barthelemy",
            "Atlantic/Bermuda",
            "Asia/Brunei",
            "America/La_Paz",
            "America/Kralendijk",
            "America/Noronha",
            "America/Belem",
            "America/Fortaleza",
            "America/Recife",
            "America/Araguaina",
            "America/Maceio",
            "America/Bahia",
            "America/Sao_Paulo",
            "America/Campo_Grande",
            "America/Cuiaba",
            "America/Santarem",
            "America/Porto_Velho",
            "America/Boa_Vista",
            "America/Manaus",
            "America/Eirunepe",
            "America/Rio_Branco",
            "America/Nassau",
            "Asia/Thimphu",
            "Africa/Gaborone",
            "Europe/Minsk",
            "America/Belize",
            "America/Blanc-Sablon",
            "America/Montreal",
            "America/Toronto",
            "America/Nipigon",
            "America/Thunder_Bay",
            "America/Iqaluit",
            "America/Pangnirtung",
            "America/Resolute",
            "America/Atikokan",
            "America/Rankin_Inlet",
            "America/Winnipeg",
            "America/Rainy_River",
            "America/Regina",
            "America/Swift_Current",
            "America/Edmonton",
            "America/Cambridge_Bay",
            "America/Yellowknife",
            "America/Inuvik",
            "America/Dawson_Creek",
            "America/Vancouver",
            "America/Whitehorse",
            "America/Dawson",
            "America/St_Johns",
            "America/Halifax",
            "America/Glace_Bay",
            "America/Moncton",
            "America/Goose_Bay",
            "Indian/Cocos",
            "Africa/Kinshasa",
            "Africa/Lubumbashi",
            "Africa/Bangui",
            "Africa/Brazzaville",
            "Europe/Zurich",
            "Africa/Abidjan",
            "Pacific/Rarotonga",
            "America/Santiago",
            "Pacific/Easter",
            "Africa/Douala",
            "Asia/Shanghai",
            "Asia/Harbin",
            "Asia/Chongqing",
            "Asia/Urumqi",
            "Asia/Kashgar",
            "America/Bogota",
            "America/Costa_Rica",
            "America/Havana",
            "Atlantic/Cape_Verde",
            "America/Curacao",
            "Indian/Christmas",
            "Asia/Nicosia",
            "Europe/Prague",
            "Europe/Berlin",
            "Africa/Djibouti",
            "Europe/Copenhagen",
            "America/Dominica",
            "America/Santo_Domingo",
            "Africa/Algiers",
            "America/Guayaquil",
            "Pacific/Galapagos",
            "Europe/Tallinn",
            "Africa/Cairo",
            "Africa/El_Aaiun",
            "Africa/Asmara",
            "Europe/Madrid",
            "Africa/Ceuta",
            "Atlantic/Canary",
            "Africa/Addis_Ababa",
            "Europe/Helsinki",
            "Pacific/Fiji",
            "Atlantic/Stanley",
            "Pacific/Chuuk",
            "Pacific/Pohnpei",
            "Pacific/Kosrae",
            "Atlantic/Faroe",
            "Europe/Paris",
            "Africa/Libreville",
            "Europe/London",
            "America/Grenada",
            "Asia/Tbilisi",
            "America/Cayenne",
            "Europe/Guernsey",
            "Africa/Accra",
            "Europe/Gibraltar",
            "America/Godthab",
            "America/Danmarkshavn",
            "America/Scoresbysund",
            "America/Thule",
            "Africa/Banjul",
            "Africa/Conakry",
            "America/Guadeloupe",
            "Africa/Malabo",
            "Europe/Athens",
            "Atlantic/South_Georgia",
            "America/Guatemala",
            "Pacific/Guam",
            "Africa/Bissau",
            "America/Guyana",
            "Asia/Hong_Kong",
            "America/Tegucigalpa",
            "Europe/Zagreb",
            "America/Port-au-Prince",
            "Europe/Budapest",
            "Asia/Jakarta",
            "Asia/Pontianak",
            "Asia/Makassar",
            "Asia/Jayapura",
            "Europe/Dublin",
            "Asia/Jerusalem",
            "Europe/Isle_of_Man",
            "Asia/Kolkata",
            "Indian/Chagos",
            "Asia/Baghdad",
            "Asia/Tehran",
            "Atlantic/Reykjavik",
            "Europe/Rome",
            "Europe/Jersey",
            "America/Jamaica",
            "Asia/Amman",
            "Asia/Tokyo",
            "Africa/Nairobi",
            "Asia/Bishkek",
            "Asia/Phnom_Penh",
            "Pacific/Tarawa",
            "Pacific/Enderbury",
            "Pacific/Kiritimati",
            "Indian/Comoro",
            "America/St_Kitts",
            "Asia/Pyongyang",
            "Asia/Seoul",
            "Asia/Kuwait",
            "America/Cayman",
            "Asia/Aqtobe",
            "Asia/Aqtau",
            "Asia/Oral",
            "Asia/Almaty",
            "Asia/Qyzylorda",
            "Asia/Vientiane",
            "Asia/Beirut",
            "America/St_Lucia",
            "Europe/Vaduz",
            "Asia/Colombo",
            "Africa/Monrovia",
            "Africa/Maseru",
            "Europe/Vilnius",
            "Europe/Luxembourg",
            "Europe/Riga",
            "Africa/Tripoli",
            "Africa/Casablanca",
            "Europe/Monaco",
            "Europe/Chisinau",
            "Europe/Podgorica",
            "America/Marigot",
            "Indian/Antananarivo",
            "Pacific/Majuro",
            "Pacific/Kwajalein",
            "Europe/Skopje",
            "Africa/Bamako",
            "Asia/Rangoon",
            "Asia/Ulaanbaatar",
            "Asia/Hovd",
            "Asia/Choibalsan",
            "Asia/Macau",
            "Pacific/Saipan",
            "America/Martinique",
            "Africa/Nouakchott",
            "America/Montserrat",
            "Europe/Malta",
            "Indian/Mauritius",
            "Indian/Maldives",
            "Africa/Blantyre",
            "America/Mexico_City",
            "America/Cancun",
            "America/Merida",
            "America/Monterrey",
            "America/Matamoros",
            "America/Mazatlan",
            "America/Chihuahua",
            "America/Ojinaga",
            "America/Hermosillo",
            "America/Tijuana",
            "America/Santa_Isabel",
            "America/Bahia_Banderas",
            "Asia/Kuala_Lumpur",
            "Asia/Kuching",
            "Africa/Maputo",
            "Africa/Windhoek",
            "Pacific/Noumea",
            "Africa/Niamey",
            "Pacific/Norfolk",
            "Africa/Lagos",
            "America/Managua",
            "Europe/Amsterdam",
            "Europe/Oslo",
            "Asia/Kathmandu",
            "Pacific/Nauru",
            "Pacific/Niue",
            "Pacific/Auckland",
            "Pacific/Chatham",
            "Asia/Muscat",
            "America/Panama",
            "America/Lima",
            "Pacific/Tahiti",
            "Pacific/Marquesas",
            "Pacific/Gambier",
            "Pacific/Port_Moresby",
            "Asia/Manila",
            "Asia/Karachi",
            "Europe/Warsaw",
            "America/Miquelon",
            "Pacific/Pitcairn",
            "America/Puerto_Rico",
            "Asia/Gaza",
            "Asia/Hebron",
            "Europe/Lisbon",
            "Atlantic/Madeira",
            "Atlantic/Azores",
            "Pacific/Palau",
            "America/Asuncion",
            "Asia/Qatar",
            "Indian/Reunion",
            "Europe/Bucharest",
            "Europe/Belgrade",
            "Asia/Novosibirsk",
            "Asia/Novokuznetsk",
            "Asia/Krasnoyarsk",
            "Asia/Irkutsk",
            "Asia/Yakutsk",
            "Asia/Vladivostok",
            "Asia/Sakhalin",
            "Asia/Magadan",
            "Asia/Kamchatka",
            "Asia/Anadyr",
            "Europe/Kaliningrad",
            "Europe/Moscow",
            "Europe/Volgograd",
            "Europe/Samara",
            "Asia/Yekaterinburg",
            "Asia/Omsk",
            "Africa/Kigali",
            "Asia/Riyadh",
            "Pacific/Guadalcanal",
            "Indian/Mahe",
            "Africa/Khartoum",
            "Europe/Stockholm",
            "Asia/Singapore",
            "Atlantic/St_Helena",
            "Europe/Ljubljana",
            "Arctic/Longyearbyen",
            "Europe/Bratislava",
            "Africa/Freetown",
            "Europe/San_Marino",
            "Africa/Dakar",
            "Africa/Mogadishu",
            "America/Paramaribo",
            "Africa/Juba",
            "Africa/Sao_Tome",
            "America/El_Salvador",
            "America/Lower_Princes",
            "Asia/Damascus",
            "Africa/Mbabane",
            "America/Grand_Turk",
            "Africa/Ndjamena",
            "Indian/Kerguelen",
            "Africa/Lome",
            "Asia/Bangkok",
            "Asia/Dushanbe",
            "Pacific/Fakaofo",
            "Asia/Dili",
            "Asia/Ashgabat",
            "Africa/Tunis",
            "Pacific/Tongatapu",
            "Europe/Istanbul",
            "America/Port_of_Spain",
            "Pacific/Funafuti",
            "Asia/Taipei",
            "Africa/Dar_es_Salaam",
            "Europe/Kiev",
            "Europe/Uzhgorod",
            "Europe/Zaporozhye",
            "Europe/Simferopol",
            "Africa/Kampala",
            "Pacific/Johnston",
            "Pacific/Midway",
            "Pacific/Wake",
            "America/Indiana/Petersburg",
            "America/Indiana/Vevay",
            "America/Chicago",
            "America/Indiana/Tell_City",
            "America/Indiana/Knox",
            "America/Menominee",
            "America/North_Dakota/Center",
            "America/North_Dakota/New_Salem",
            "America/North_Dakota/Beulah",
            "America/Denver",
            "America/Boise",
            "America/Shiprock",
            "America/Phoenix",
            "America/Los_Angeles",
            "America/Anchorage",
            "America/Juneau",
            "America/Sitka",
            "America/Yakutat",
            "America/Nome",
            "America/Adak",
            "America/Metlakatla",
            "Pacific/Honolulu",
            "America/New_York",
            "America/Detroit",
            "America/Kentucky/Louisville",
            "America/Kentucky/Monticello",
            "America/Indiana/Indianapolis",
            "America/Indiana/Vincennes",
            "America/Indiana/Winamac",
            "America/Indiana/Marengo",
            "America/Montevideo",
            "Asia/Samarkand",
            "Asia/Tashkent",
            "Europe/Vatican",
            "America/St_Vincent",
            "America/Caracas",
            "America/Tortola",
            "America/St_Thomas",
            "Asia/Ho_Chi_Minh",
            "Pacific/Efate",
            "Pacific/Wallis",
            "Pacific/Apia",
            "Asia/Aden",
            "Indian/Mayotte",
            "Africa/Johannesburg",
            "Africa/Lusaka",
            "Africa/Harare"
        };
        #endregion
    }

    [DataContract]
    public class TimeModel : IComparable<TimeModel>
    {
        public TimeModel()
        {
        }
        [DataMember]
        public string CountryCode { get; set; }
        [DataMember]
        public string ZoneName { get; set; }
        [DataMember]
        public string Abbreviation { get; set; }
        [DataMember]
        public int GMTOffset { get; set; }
        [DataMember]
        public bool DaylightSaving { get; set; }
        [DataMember]
        public double TimeStamp { get; set; }

        public bool IsError { get; set; }

        public override bool Equals(object obj)
        {
            if (obj == null)
                return false;
            TimeModel other = obj as TimeModel;
            if (other == null)
                return false;

            return  this.CountryCode        == other.CountryCode        &&
                    this.ZoneName           == other.ZoneName           &&
                    this.Abbreviation       == other.Abbreviation       &&
                    this.GMTOffset          == other.GMTOffset          &&
                    this.DaylightSaving     == other.DaylightSaving     &&
                    this.TimeStamp          == other.TimeStamp;
        }

        public override int GetHashCode()
        {
            unchecked
            {
                int hash = 17;
                int salt = 29;

                hash = hash * salt + this.CountryCode.GetHashCode();
                hash = hash * salt + this.ZoneName.GetHashCode();
                hash = hash * salt + this.Abbreviation.GetHashCode();
                hash = hash * salt + this.GMTOffset.GetHashCode();
                hash = hash * salt + this.DaylightSaving.GetHashCode();
                hash = hash * salt + this.TimeStamp.GetHashCode();

                return hash;
            }
        }

        int IComparable<TimeModel>.CompareTo(TimeModel other)
        {
            return this.ZoneName.CompareTo(other.ZoneName);
        }
    }
}