using System;
using System.Collections.Generic;
using System.IO;
using System.Net.Http;
using System.Threading.Tasks;

namespace TestApp
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            Task.Run(() => new Downloader().Download(
                "url_file_download",
                "url_save_to"
                )).Wait();
        }
    }

    public class Downloader
    {
        public async Task Download(string url, string saveAs)
        {
            var httpClient = new HttpClient();
            var response = await httpClient.SendAsync(new HttpRequestMessage(HttpMethod.Head, url));
            var parallelDownloadSuported = response.Headers.AcceptRanges.Contains("bytes");
            var contentLength = response.Content.Headers.ContentLength ?? 0;

            if (parallelDownloadSuported)
            {
                const double numberOfParts = 5.0;
                var tasks = new List<Task>();
                var partSize = (long)Math.Ceiling(contentLength / numberOfParts);

                File.Create(saveAs).Dispose();

                for (var i = 0; i < numberOfParts; i++)
                {
                    var start = i*partSize + Math.Min(1, i);
                    var end = Math.Min((i + 1)*partSize, contentLength);

                    tasks.Add(
                        Task.Run(() => DownloadPart(url, saveAs, start, end))
                        );
                }

                await Task.WhenAll(tasks);
            }
        }

        private async Task DownloadPartAsync(string url, string saveAs, long start, long end)
        {
            using (var httpClient = new HttpClient())
            using (var fileStream = new FileStream(saveAs, FileMode.Open, FileAccess.Write, FileShare.Write))
            {
                var message = new HttpRequestMessage(HttpMethod.Get, url);
                message.Headers.Add("Range", string.Format("bytes={0}-{1}", start, end));

                fileStream.Position = start;
                var response = await httpClient.SendAsync(message);
                await response.Content.CopyToAsync(fileStream);
            }
        }
    }
}