An easy-to-use URL file downloader class for .NET

原文地址: http://www.codeproject.com/KB/IP/filedownloader.aspx

 

• Download source and demo project (.NET 2.0) - 145.7 KB
• Download source and demo project (.NET 1.1) - 117.9 KB

Introduction

For one of my projects at work, I needed the ability to download a file from a given URL. Unfortunately, the methods provided by the WebClient class don't do anything more than download the data. I needed something that gave the ability to provide progress feedback and that was resumable.

As I started my searches on the web for any existing code, I came across two articles on CodeProject that helped get me very close to what I needed. The first article was by John Batte, while the second article by Phil Crosby was a derivation of his work. Using both of these articles as a starting point, I was able to make another derivation that accomplished my goals.

The code samples and content of this article are focused on the .NET 2.0 implementation and any code updates have been made only to that version. The .NET 1.1 implementation is provided for backwards compatibility only and I have no further plans to continue making changes to it.

Using the Code

The FileDownloader class is the main class and is very simple to use. The minimal implementation is:

Collapse

private void DoDownload()
{
    FileDownloader downloader = new FileDownloader();
    downloader.Download(new Uri("http://www.codeproject.com" +
        "/info/stuff/cp_bg_black1024.gif"), @"C:/Temp");
}

This creates a new instance of the FileDownloader class and downloads the file located at "http://www.codeproject.com/info/stuff/cp_bg_black1024.gif" to the "C:/Temp" folder.

FileDownloader actually performs all download operations asynchronously. When the FileDownloader.Download(Uri url, string destinationFolder) method is called, it accomplishes a synchronous download by waiting until a signal is set indicating that the download has completed. This allows for both synchronous downloads -- through the use of the Download methods -- or asynchronous downloads, through the use of the DownloadAsync methods. Both the synchronous and asynchronous methods ultimately call the same internal method to perform the actual download.

This internal method, called DownloadAsyncCore, instantiates an instance of the DownloadInfo internal class. This class is similar in nature to the FileInfo class provided by the .NET CLR and provides FileDownloader with all of the information required to actually download the file. This includes:

• The length of the file being downloaded, if known
• The Stream that is being used to actually download the file
• The WebResponse that is servicing the request

Once the DownloadInfo class is instantiated, the data is downloaded in 1KB chunks and written to the disk. This is the default block size and can be changed through the BlockSize property. It is this implementation that allows the download to resume if it is cancelled or interrupted.

While this implementation is functional, it does prevent anyone from using the class who is behind a proxy or who requires authentication to access the requested URL. In order to support these scenarios, the FileDownloader class provides both a Proxy property that accepts an IWebProxy derived object and a Credentials property that accepts an ICredentials derived object. These properties are also passed on to the underlying DownloadInfo object.

Finally, the events provide progress feedback. The WebClient class in .NET 2.0 has asynchronous download methods that provide just such events. These methods are serviced by the DownloadCompletedEventArgs and DownloadProgressChangedEventArgs EventArgs derived classes. Unfortunately, both of these classes have internal constructors and are new in .NET 2.0, so I wasn't able to make use of them. Instead, I created a new triplet set of EventArgs derived classes to handle the three events that are exposed by the FileDownloader class. These events are:

• public event EventHandler<FileDownloadCompletedEventArgs> DownloadCompleted
• public event EventHandler<FileDownloadProgressChangedEventArgs> DownloadProgressChanged;
• public event EventHandler<FileDownloadStatusChangedEventArgs> DownloadStatusChanged;

These events are raised during the actual file download to provide the caller with status information (DownloadStatusChanged), progress information (DownloadProgressChanged) and completion information (DownloadCompleted). A complete example looks like this:

Collapse

private void DoDownload()
{
    Uri fileUrl = new Uri("http://www.codeproject.com" +
        "/info/stuff/cp_bg_black1024.gif");
    string tempPath = Path.GetTempPath();
    string fileName = Path.GetFileName(fileUrl.AbsoluteUri);
    string downloadPath = Path.Combine(tempPath, fileName);

    FileDownloader downloader = new FileDownloader();
    downloader.Proxy = WebRequest.DefaultWebProxy;
    downloader.Credentials = CredentialCache.DefaultCredentials;

    downloader.DownloadCompleted += new
        EventHandler<FileDownloadCompletedEventArgs>(OnDownloadCompleted);
    downloader.DownloadProgressChanged += new
        EventHandler<FileDownloadProgressChangedEventArgs>(
        OnDownloadProgressChanged);
    downloader.DownloadStatusChanged += new
        EventHandler<FileDownloadStatusChangedEventArgs>(
        OnDownloadStatusChanged);

    // Download the requested file to the specified folder

    downloader.Download(fileUrl, tempPath);
}

private void OnDownloadCompleted(object sender,
    FileDownloadCompletedEventArgs e)
{
    // Do something when the download completes.

}

private void OnDownloadProgressChanged(object sender,
    FileDownloadProgressChangedEventArgs e)
{
    // Do something when the progress changes,

    // usually you would increment a progress bar.

}

private void OnDownloadStatusChanged(object sender,
    FileDownloadStatusChangedEventArgs e)
{
    // Do something when that status changes.

}

Here is the same example using VB.NET. This example is based on the comment [^] by CBlackburn [^].

Collapse

Private Sub DoDownload()
    Dim fileUrl As Uri = New Uri("http://www.codeproject.com/" & _
        "info/stuff/cp_bg_black1024.gif")

    Dim tempPath As String = Path.GetTempPath()
    Dim fileName As String = Path.GetFileName(fileUrl.AbsoluteUri)
    Dim downloadPath As String = Path.Combine(tempPath, fileName)

    Dim downloader As FileDownloader = New FileDownloader()
    downloader.Proxy = WebRequest.DefaultWebProxy
    downloader.Credentials = CredentialCache.DefaultCredentials

    AddHandler downloader.DownloadCompleted, _
        AddressOf OnDownloadCompleted
    AddHandler downloader.DownloadProgressChanged, _
        AddressOf OnDownloadProgressChanged
    AddHandler downloader.DownloadStatusChanged, _
        AddressOf OnDownloadStatusChanged

    downloader.Download(fileUrl, tempPath)
End Sub

Private Sub OnDownloadCompleted(ByVal sender As Object, _
    ByVal e As FileDownloadCompletedEventArgs)
    ' Do something when the download completes.

End Sub

Private Sub OnDownloadProgressChanged(ByVal sender As Object, _
    ByVal e As FileDownloadProgressChangedEventArgs)
    ' Do something when the progress changes,

    ' usually you would increment a progress bar.

End Sub

Private Sub OnDownloadStatusChanged(ByVal sender As Object, _
    ByVal e As FileDownloadStatusChangedEventArgs)
    ' Do something when that status changes.

End Sub

The complete public API for the FileDownloader class is available in the documentation CHM file that is part of the download.

Points of Interest

While the FileDownloader class is available as both a .NET 2.0 and .NET 1.1 solution, only the .NET 2.0 solution has been updated to provide the asynchronous download features and other modifications listed in the revision history.

The DownloadInfo class implements the IDisposable pattern. However, the way this pattern is implemented may be different than many of you have seen before. For more information, please refer to this article: Implementing IDisposable and the Dispose Pattern Properly [^] .