using System;
    using System.Threading.Tasks;
    using System.Collections.Generic;
    using static System.Console;

    namespace tasks
    {
    public class Program
    {
        static void Main(string[] args)
        {
            WriteLine("** Start");
            WriteLine("** Update Data");

            var requester = new DataRequester();
            requester.UpdateData();

            WriteLine("** Ending...");
            Console.ReadLine();
        }
    }
    public class DataEventArgs: EventArgs
    {
        public string FileName {get; set;}
        public List<string> Data {get; set;}
        public DataEventArgs(string fileName, List<string> data)
        {
            FileName = fileName;
            Data = data;
        }
    }
    public class DataRequester
    {
        public void UpdateData()
        {
            // The real code loads fileNames from a CSV File
            var numberOfCSVFiles = 1000;
            var fileNames = new string[numberOfCSVFiles];
            var loader = new DataLoader(fileNames);
            loader.OnDataLoaded += HandleDataLoaded;
            loader.OnDataNotFound += HandleDataNotFound;
            loader.RequestData();
        }
        public void HandleDataLoaded(object sender, DataEventArgs args)
        {
            WriteLine($"Received the data for file: {args.FileName}");

            // Update SQLite Database ...
        }
        public void HandleDataNotFound(object sender, DataEventArgs args)
        {
            WriteLine($"Data not found for file: {args.FileName}");
        }
    }
    public class DataLoader
    {
        public event EventHandler<DataEventArgs> OnDataLoaded;
        public event EventHandler<DataEventArgs> OnDataNotFound;
        public string[] FileNames {get; set;}
        public DataLoader(string[] fileNames){FileNames = fileNames;}
        private bool CheckFileName(string fileName)
        {
            var random = new Random();
            var fileExists = (random.Next(4)<3)?true:false;
            return fileExists;
        }
        private DataEventArgs LoadData(string fileName)
        {
            // Simulate data loading
            var random = new Random();
            Task.Delay(random.Next(10)*1000).Wait();
            return  new DataEventArgs(fileName, new List<string>());
        }
        private Task<DataEventArgs> LoadDataAsync(string fileName)
        {
            return Task.Run<DataEventArgs>(() =>
            {
                return LoadData(fileName);
            });
        }
        public async void RequestData()
        {
            var tasks = new List<Task>();

            for(int i=0; i<FileNames.Length; i++)
            {
                var fileName = String.Concat("file #",(i+1).ToString());
                if(CheckFileName(fileName))
                {
                    var task = LoadDataAsync(fileName).ContinueWith((t) =>
                    {
                        OnDataLoaded(this, t.Result);
                    });

                    tasks.Add(task);
                }
                else
                {
                    var args = new DataEventArgs(fileName, new List<string>());
                    OnDataNotFound?.Invoke(this, args);
                }
            }

            await Task.WhenAll(tasks);
            WriteLine("Request completed.");
        }
    }
}