SumNumbers

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Threading.Tasks;

namespace sum_files_solution
{
    class Program
    {
        static async Task Main(string[] args)
        {
            List<Task<long>> tasks = new List<Task<long>>();

            var files = Directory.EnumerateDirectories("files")
                .SelectMany(directory => Directory.EnumerateFiles(directory));

            foreach (var file in files)
            {
                var task = Task.Run(() => SumFile(file));
                tasks.Add(task);
            }

            var result = await Task.WhenAll(tasks);
            var sumOfFiles = result.Sum();

            long SumFile(string file)
            {
                long sum = 0;
                foreach (var line in File.ReadLines(file))
                {
                    foreach (var shard in line.Split(','))
                        sum += int.Parse(shard);
                }

                return sum;
            }

            // Total: 49947871404
            Console.WriteLine("Total: {0}", sumOfFiles);
        }
    }
}