C# isfileready without freezeing gui

Question

I have a created a WPF gui (netframework 3.5) that is used to download files. Some of them come archived and I would like to have them automatically extracted after the download is complete. As the internet speed will vary from system to system, the only wait to tell when the file is completed is to use a IsFileReady method I have found here. The problem with it? It freezes my UI. Any way to bypass this issue, while remaining on netframework 3.5 based project?

I used the next piece of code and even modified it a bit with a background worker, but no go. still freezes UI

public static bool IsFileReady(string filename)
        {

            try
            {
                using (FileStream inputStream = File.Open(filename, FileMode.Open, FileAccess.Read, FileShare.None))
                    return inputStream.Length > 0;
            }
            catch (Exception)
            {
                return false;
            }
        }
        public static void WaitForFile(string filename)
        {
            BackgroundWorker tb = new BackgroundWorker();
            tb.DoWork += new DoWorkEventHandler(delegate (object o, DoWorkEventArgs args)
            {
                while (!IsFileReady(filename)) ;
            });
            tb.RunWorkerAsync();

        }

Finally used WaitForFile function inside a button press method. It does the job, but as I said it freezes the UI.

Answer 1

You want to download a file asynchronously but you're restricted to .NET Framework 3.5 and so don't have access to Task? In that case, you don't need the IsFileReady function and you might not need the BackgroundWorker. System.Net.WebClient provides methods for downloading files asynchronously. Namely, System.Net.WebClient.DownloadFileAsync to do the download, and System.Net.WebClient.DownloadFileCompleted to report when the download has completed.

// MainWindow.xaml
<Window x:Class="DownloadFileAsync.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <StackPanel>
      <TextBlock>Uri:</TextBlock>
      <TextBox x:Name="UriTxt"></TextBox>
      <TextBlock>File Name:</TextBlock>
      <TextBox x:Name="FileNameTxt"></TextBox>
      <Label x:Name="StatusLbl">Waiting...</Label>
      <Button x:Name="DownloadBtn" Click="DownloadBtn_Click">Download</Button>
    </StackPanel>
</Window>

// MainWindow.xaml.cs
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.IO;
using System.Linq;
using System.Net;
using System.Text;
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;

namespace DownloadFileAsync
{
   /// <summary>
   /// Interaction logic for MainWindow.xaml
   /// </summary>
   public partial class MainWindow : Window
   {
      public MainWindow()
      {
         InitializeComponent();

         _client = new System.Net.WebClient();
         _client.DownloadFileCompleted += _client_DownloadFileCompleted;

         // I was using https://github.com/danielmiessler/SecLists/raw/master/Passwords/bt4-password.txt as the file I was testing which
         // required TLS1.2. This line enables TLS1.2, you may or may not need it.
         ServicePointManager.SecurityProtocol = (SecurityProtocolType)0x00000C00;
      }

      void _client_DownloadFileCompleted(object sender, System.ComponentModel.AsyncCompletedEventArgs e)
      {
         if (e.Error != null)
         {
            // If there was an error downloading the file, show it.
            MessageBox.Show(e.Error.Message);
         }
         else if (e.Cancelled)
         {
            StatusLbl.Content = "Cancelled!";
         }
         else
         {
            // There weren't any errors and the download wasn't cancelled, so then it must have completed successfully.
            // If the file is an archive, this is where you could extract it. Do be aware that you're back on the UI thread
            // at this point, so the UI will block while the file is extracting.
            //
            // If extraction is going to take some time and you don't want it to block, you'll either need to use an async
            // API to extract the archive or start a BackgroundWorker here to extract the archive.
            StatusLbl.Content = "File Downloaded!";
         }
         DownloadBtn.IsEnabled = true;
      }

      private void DownloadBtn_Click(object sender, RoutedEventArgs e)
      {
         if (string.IsNullOrEmpty(FileNameTxt.Text))
         {
            MessageBox.Show("You must enter a file name");
            return;
         }
         Uri uri;
         if (!Uri.TryCreate(UriTxt.Text, UriKind.Absolute, out uri))
         {
            MessageBox.Show("You must enter a valid uri");
            return;
         }
         StatusLbl.Content = "Downloading...";
         DownloadBtn.IsEnabled = false;
         _client.DownloadFileAsync(uri, FileNameTxt.Text);
      }

      private readonly System.Net.WebClient _client;
   }
}

I made a note of it in the comments, but the delegate you provide to DownloadFileCompleted is executing on the UI thread. So, if you need to extract an archive there the UI will block until the extraction is complete. You'll either need to use an asynchronous API for extracting the archive, or start a BackgroundWorker to extract the archive if you need to avoid blocking the UI during the extraction.