Download multiple files in parallel using c#

Question

I want to download files in parallel using C#. For this, I have written this code which is working perfectly but the problem is that UI is freezing.

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Windows.Threading;

namespace FileDownloader
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        private static int count = 1;
        private static string f= "lecture";
        private string URL = "www.someexample.com";

        public MainWindow()
        {
            InitializeComponent();
        }

        public static string GetDirectoryListingRegexForUrl(string url)
        {
            if (url.Equals(URL))
            {
                return "<a href=\".*\">(?<name>.*)</a>";
            }
            throw new NotSupportedException();
        }

        public  void DownloadP(string[] urls)
        {
            Parallel.ForEach(urls.ToList(), new ParallelOptions { MaxDegreeOfParallelism = 10 }, DownloadFile);
        }

        private void DownloadFile(string url)
        {
           using(WebClient client=new WebClient())
           {
               if (url.EndsWith(".pdf"))
               {
                   int nextIndex = Interlocked.Increment(ref count);

                   client.DownloadFile(url, f + nextIndex + ".pdf");
                   this.Dispatcher.Invoke(() => {
                       listbox.Items.Add(url);
                   });
               }
           }
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            DownloadP(listofFiles);
        }
    }
}

Answer 1

You can use async/await with conjunction with new WebClient method DownloadFileTaskAsync.

private async Task DownloadFile(string url)
{
    if (!url.EndsWith(".pdf"))
    {
        return;
    }

    using (var client = new WebClient())
    {
        int nextIndex = Interlocked.Increment(ref count);

        await client.DownloadFileTaskAsync(url, "lecture" + nextIndex + ".pdf");
        listBox.Items.Add(url);

    }
}

private async void Button_OnClick(object sender, RoutedEventArgs e)
{
    button.IsEnabled = false;
    await DownloadFiles(urlList);
    button.IsEnabled = true;
}

private async Task DownloadFiles(IEnumerable<string> urlList)
{
    foreach (var url in urlList)
    {
        await DownloadFile(url);
    }
}

Answer 2

Relace your DownloadP function with this :

public async Task DownloadP(string[] urls)
{
  await Task.Factory.StartNew(() => Parallel.ForEach(urls.ToList(), new ParallelOptions { MaxDegreeOfParallelism = 10 }, DownloadFile));
}

Answer 3

instead of using client.DownloadFile use client.DownloadFileAsync like this

var webClient=new WebClient();
webClient.DownloadFileCompleted += webClient_DownloadFileCompleted;
webClient.DownloadFileAsync("Your url","file_name");

the event

    private void webClient_DownloadFileCompleted(object sender, AsyncCompletedEventArgs e)
    {
        MessageBox.Show("Download Completed.");
    }