Downloading Files in .NET With All Information: Progressbar, Download Speed, Supports Cancel and Resume

• Download demo - 10.41 KB
• Download source files - 12.56 KB

Introduction  

I'll explain how you can build a complete, reliable and powerful download manager with .NET Framework.
It is able to show a rarity of data about the download in progress, including the download speed, the file size, the downloaded size, and the percentage of completion.

A completely new class has been written based on this code, which features logic/layout abstraction, multiple file download support, and is more easy to use by providing a variety of properties and events. Check out the FileDownloader article.

The files are downloaded via the HttpWebRequest  and HttpWebResponse, and written to a file with the StreamWriter. All this is done on a separate thread (with a BackgroundWorker), so the application using this downloader won't freeze.

Using the Code 

The demo application is pretty simple, and contains only the needed UI components for downloading a file. You can of course implement this downloader in your own application, or just use it to download in the background without any user interaction.

The most important code can be found in the DoWork method of the BackgroundWorker. First, an attempt will be made to establish a connection to the file. If this is successful, a script in the Do loop will start downloading the file block by block, until there are no remaining bytes to be downloaded. Note that as soon as a block of data has been received, it's written to the local target file.

Private Sub BackgroundWorker1_DoWork(ByVal sender As System.Object, _
    ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork

        'Creating the request and getting the response
        Dim theResponse As HttpWebResponse
        Dim theRequest As HttpWebRequest
        Try 'Checks if the file exist

            theRequest = WebRequest.Create(Me.txtFileName.Text)
            theResponse = theRequest.GetResponse
        Catch ex As Exception

            MessageBox.Show("An error occurred while downloading file. _
			Possible causes:" & ControlChars.CrLf & _
                            "1) File doesn't exist" & ControlChars.CrLf & _
                            "2) Remote server error", "Error", _
			MessageBoxButtons.OK, MessageBoxIcon.Error)

            Dim cancelDelegate As New DownloadCompleteSafe(AddressOf DownloadComplete)

            Me.Invoke(cancelDelegate, True)

            Exit Sub
        End Try
        Dim length As Long = theResponse.ContentLength 'Size of the response (in bytes)

        Dim safedelegate As New ChangeTextsSafe(AddressOf ChangeTexts)
        Me.Invoke(safedelegate, length, 0, 0, 0) 'Invoke the TreadsafeDelegate

        Dim writeStream As New IO.FileStream(Me.whereToSave, IO.FileMode.Create)

        'Replacement for Stream.Position (webResponse stream doesn't support seek)
        Dim nRead As Integer

        'To calculate the download speed
        Dim speedtimer As New Stopwatch
        Dim currentspeed As Double = -1
        Dim readings As Integer = 0

        Do

            If BackgroundWorker1.CancellationPending Then 'If user aborts download
                Exit Do
            End If

            speedtimer.Start()

            Dim readBytes(4095) As Byte
            Dim bytesread As Integer = theResponse.GetResponseStream.Read_
						(readBytes, 0, 4096)

            nRead += bytesread
            Dim percent As Short = (nRead * 100) / length

            Me.Invoke(safedelegate, length, nRead, percent, currentspeed)

            If bytesread = 0 Then Exit Do

            writeStream.Write(readBytes, 0, bytesread)

            speedtimer.Stop()

            readings += 1
            If readings >= 5 Then 'For increase precision, _
			' the speed is calculated only every five cycles
                currentspeed = 20480 / (speedtimer.ElapsedMilliseconds / 1000)
                speedtimer.Reset()
                readings = 0
            End If
        Loop

        'Close the streams
        theResponse.GetResponseStream.Close()
        writeStream.Close()

        If Me.BackgroundWorker1.CancellationPending Then

            IO.File.Delete(Me.whereToSave)

            Dim cancelDelegate As New DownloadCompleteSafe(AddressOf DownloadComplete)

            Me.Invoke(cancelDelegate, True)

            Exit Sub

        End If

        Dim completeDelegate As New DownloadCompleteSafe(AddressOf DownloadComplete)

        Me.Invoke(completeDelegate, False)

End Sub

This code uses delegates to invoke methods on the main thread, and pass data about the download progress to them. 

The code does not support FTP downloads, but can be easily modified to do this.  

Resume Downloads 

It is possible to modify the code to allow resuming downloads. Add this code before the first use of the HttpWebRequest object.

theRequest.AddRange(whereYouWantToStart) '<- add this

You'll also need to set the Position property of the FileStream instance to the position where you want to resume the download. So be sure you also save this before the download is cancelled.

Dim writeStream As New IO.FileStream(Me.whereToSave, IO.FileMode.Open)
writeStream.Position = whereYouWantToStart

Calculating Speed 

The code contains built in download speed calculation, using the StopWatch class. This will return another result every time 5 packages have been downloaded. Note that you can also manually calculate the speed, and use a timer to get data at a more regular interval.