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Is there any way i can pause a file download in my vb program?? I have tried both the http method and the my.computer.net method with no luck. I have also tried pausing the background worker by this method: [URL] But even if the bgworker is paused the download goes on.....
 
Dim locationfiledownload As String
Dim whereToSave As String 'Where the program save the file

Delegate Sub ChangeTextsSafe(ByVal length As Long, ByVal position As Integer, ByVal percent As Integer, ByVal speed As Double)
Delegate Sub DownloadCompleteSafe(ByVal cancelled As Boolean)
 
Public Sub DownloadComplete(ByVal cancelled As Boolean)
    ToolStripButton2.Enabled = True
    ToolStripButton3.Enabled = False
 
    If cancelled Then
        Me.Label4.Text = "Cancelled"
        MessageBox.Show("Download aborted", "Aborted", MessageBoxButtons.OK, MessageBoxIcon.Information)
    Else
        Me.Label4.Text = "Successfully downloaded"
        MessageBox.Show("Successfully downloaded!", "All OK", MessageBoxButtons.OK, MessageBoxIcon.Information)
    End If
 
    Me.RadProgressBar1.Value1 = 0
    Me.Label4.Text = ""
 
End Sub
 
Public Sub ChangeTexts(ByVal length As Long, ByVal position As Integer, ByVal percent As Integer, ByVal speed As Double)
    Me.Label4.Text = "Downloaded " & Math.Round((position / 1024), 2) & " KB of " & Math.Round((length / 1024), 2) & "KB"
    Me.RadProgressBar1.Value1 = percent
End Sub
 
Public Sub btnDownload_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ToolStripButton2.Click
    locationfiledownload = GetPage("http://asankonkur.ir/update/locationfiledownload.txt")
 
    If locationfiledownload <> "" AndAlso locationfiledownload.StartsWith("http://") Then
 
        Me.SaveFileDialog1.FileName = locationfiledownload.Split("/"c)(locationfiledownload.Split("/"c).Length - 1)
        If Me.SaveFileDialog1.ShowDialog = Windows.Forms.DialogResult.OK Then
            Me.whereToSave = Me.SaveFileDialog1.FileName
            Me.SaveFileDialog1.FileName = ""
 
            ToolStripButton2.Enabled = False
            ToolStripButton3.Enabled = True
 
            Me.BackgroundWorker2.RunWorkerAsync() 'Start download
        End If
    Else
        MessageBox.Show("Please insert valid URL for download", "Warning", MessageBoxButtons.OK, MessageBoxIcon.Warning)
    End If
End Sub
 
Private Sub BackgroundWorker2_DoWork(ByVal sender As System.Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker2.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(locationfiledownload)
        theResponse = theRequest.GetResponse
    Catch ex As Exception
 
        MessageBox.Show("An error occurred while downloading file. Possibe 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 BackgroundWorker2.CancellationPending Then 'If user abort 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 it's calculated only every five cicles
            currentspeed = 20480 / (speedtimer.ElapsedMilliseconds / 1000)
            speedtimer.Reset()
            readings = 0
        End If
    Loop
    'Close the streams
    theResponse.GetResponseStream.Close()
    writeStream.Close()
    If Me.BackgroundWorker2.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
 
Private Sub mainForm_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    Me.Label4.Text = ""
End Sub
 
Private Sub btnCancel_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ToolStripButton3.Click
    Me.BackgroundWorker2.CancelAsync() 'Send cancel request
End Sub
Posted 15-Feb-13 6:38am

1 solution

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Solution 1

For pausing/resuming thread, use the class System.Threading.EventWaitHandle or System.Threading.ManualResetEvent (the behavior of these classes are exactly identical in case you use manual reset). In a thread, you use System.Threading.EventWaitHandle.WaitOne. This is a blocking code. If the event is reset (is in nonsignaled state), it will switch out your downloading thread and put it in the wait state, that is, the thread won't be scheduled back to execution until awaken. So, the wait spends zero CPU time. The thread will be awaken by Thread.Abort or when the same instance EventWaitHandle is signaled, which you can do in some different (say, UI) thread, by calling EventWaitHandle.Set. This is your mechanism for suspending/resuming your thread.
 
Obsolete methods Suspend/Resume are unsafe and should be be used.
 
As to downloading, I would rather advise to use System.Net.HttpWebRequest. Please see:
http://msdn.microsoft.com/en-us/library/system.net.webrequest.aspx[^],
http://msdn.microsoft.com/en-us/library/system.net.httpwebrequest.aspx[^].
 
In contrast to other method, this class will allow you to download a file sequentially, piece by piece, so you can insert EventWaitHand.WaitOne between blocks. More high-level (less direct) methods based on events won't allow you to do so. For a complete code sample, please see my application HttpDownloader:
how to download a file from internet[^].
 
Please see also my other past answers on the topic:
How to get the data from another site[^],
get specific data from web page[^].
 
—SA
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