Here’s a rundown of how the asynchronous model works in this instance:
Listen to events that will notify you of the download status.
Start the download event with an -Async method.
Do other stuff (even if it’s just listen for a button click to cancel).
In the event handlers for the download events, respond to the download progress (or completion).
Listing 1
shows a portion of the code for our sample application.
Listing 1. Asynchronous Web Downloader
using System;
using System.ComponentModel;
using System.Text;
using System.Windows.Forms;
using System.Net;
namespace WebDownloaderAsync
{
public partial class Form1 : Form
{
WebClient _client = null;
bool _downloading = false;//for tracking what button does
public Form1()
{
InitializeComponent();
}
private void buttonDownload_Click(object sender, EventArgs e)
{
if (!_downloading)
{
_client = new WebClient();
//listen for events so we know when things happen
_client.DownloadProgressChanged +=
_client_DownloadProgressChanged;
_client.DownloadDataCompleted +=
_client_DownloadDataCompleted;
try
{
//start downloading and immediately return
_client.DownloadDataAsync(new Uri(textBoxUrl.Text));
//now our program can do other stuff while we wait!
_downloading = true;
buttonDownload.Text = "Cancel";
}
catch (UriFormatException ex)
{
MessageBox.Show(ex.Message);
_client.Dispose();
}
catch (WebException ex)
{
MessageBox.Show(ex.Message);
_client.Dispose();
}
}
else
{
_client.CancelAsync();
}
}
void _client_DownloadProgressChanged(object sender,
DownloadProgressChangedEventArgs e)
{
progressBar.Value = e.ProgressPercentage;
labelStatus.Text =
string.Format("{0:N0} / {1:N0} bytes received",
e.BytesReceived,
e.TotalBytesToReceive);
}
void _client_DownloadDataCompleted(object sender, DownloadDataCompletedEventArgs e)
{
//now the file is done downloading
if (e.Cancelled)
{
progressBar.Value = 0;
labelStatus.Text = "Cancelled";
}
else if (e.Error != null)
{
progressBar.Value = 0;
labelStatus.Text = e.Error.Message;
}
else
{
progressBar.Value = 100;
labelStatus.Text = "Done!";
}
//don't forget to dispose our download client!
_client.Dispose();
_downloading = false;
buttonDownload.Text = "Download";
//access data in e.Result
}
}
}
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