The main problems (set Option Strict On to find more):
You can’t access the UI objects from a thread different than the UI Thread.
The error you receive is:
Cross-thread operation not valid:
ControllblFileSizeDaccessed from
a thread other than the thread it was created on
Then, the same error for lblDownloadedBytesD.
Also, you are eating up your Error messages using an empty handler with:
Catch ex As Exception
End Try
This nullifies any handling, because there’s none. You are simply letting the code run past it without taking any action. The handlers are there to, well, handle the errors, not to let them go unchecked.
When you need to access and update some UI component property, use the BackGroundWorker ReportProgress() method. This method has an overload that accepts a parameter of type Object. Meaning, you can feed it anything. This Object will be the e.UserState property in the ReportProgress ProgressChangedEventArgs class.
The .RunWorkerAsync() method also accepts an Object parameter. This Object will become the e.Argument property of the BackgroundWorker.DoWork Event. This gives some flexibility in relation to the parameters you can actually pass to your BackGroundWorker.
One more problem: the Ftp Download procedure does not support cancellation. When run, a user can’t stop it.
Last problem: as reported in the documentation, you should never reference the BackGroundWorker object you instantiated in your UI thread (the Form) in its DoWork event. Use the sender object and cast it to the BackGroundWorker class.
In this example, all the UI references are delegated to a Class object that is passed to the DoWork event through the RunWorkerAsync(Object) method (using the e.Argument property).
The Class object is updated with progress details and then fed to the ReportProgress(Int32, Object) method, which runs in the original Synchronization Context (the UI thread, where the RunWorkerAsync method is called).
The UI can be updated safely. No cross-thread operations can occur.
A cancellation method is also implemented. This allows to abort the download procedure and to delete a partial downloaded file, if one is created.
The error handling is minimal, but this is something you need to integrate with your own tools.
(I’ve used the same names for the UI Controls, it should be easier to test.)
Imports System.ComponentModel
Imports System.Globalization
Imports System.IO
Imports System.Net
Imports System.Net.Security
Imports System.Security.Cryptography.X509Certificates
Public Class frmBGWorkerDownload
Friend WithEvents BWorkerD As BackgroundWorker
Public Sub New()
InitializeComponent()
BWorkerD = New BackgroundWorker()
BWorkerD.WorkerReportsProgress = True
BWorkerD.WorkerSupportsCancellation = True
AddHandler BWorkerD.DoWork, AddressOf BWorkerD_DoWork
AddHandler BWorkerD.ProgressChanged, AddressOf BWorkerD_ProgressChanged
AddHandler BWorkerD.RunWorkerCompleted, AddressOf BWorkerD_RunWorkerCompleted
BWorkerD.RunWorkerAsync(BGWorkerObj)
End Sub
Private Class BGWorkerObject
Public Property UserName As String
Public Property Password As String
Public Property ResourceURI As String
Public Property FilePath As String
Public Property FileLength As Long
Public Property DownloadedBytes As Long
Public Property BytesToDownload As Long
End Class
Private Sub btnDownload_Click(sender As Object, e As EventArgs) Handles btnDownload.Click
pBarD.Value = 0
Dim BGWorkerObj As BGWorkerObject = New BGWorkerObject With {
.ResourceURI = txtFilePathD.Text,
.FilePath = Path.Combine(txtSavePathD.Text, Path.GetFileName(txtFilePathD.Text)),
.UserName = txtFTPUsernameD.Text,
.Password = txtFTPPasswordD.Text
}
End Sub
Private Sub BWorkerD_DoWork(sender As Object, e As DoWorkEventArgs)
Dim BGW As BackgroundWorker = TryCast(sender, BackgroundWorker)
Dim BGWorkerObj As BGWorkerObject = TryCast(e.Argument, BGWorkerObject)
Dim FTPRequest As FtpWebRequest
Dim BufferSize As Integer = 131072
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls11 Or SecurityProtocolType.Tls12
ServicePointManager.ServerCertificateValidationCallback =
Function(snd As Object, Cert As X509Certificate, Chain As X509Chain, Err As SslPolicyErrors)
Return True
End Function
FTPRequest = DirectCast(WebRequest.Create(BGWorkerObj.ResourceURI), FtpWebRequest)
FTPRequest.Credentials = New NetworkCredential(BGWorkerObj.UserName, BGWorkerObj.Password)
'FTPRequest.Method = WebRequestMethods.Ftp.GetFileSize
'----------------------- UPDATE ------------------------
FTPRequest.Method = WebRequestMethods.Ftp.ListDirectoryDetails
'--------------------- END UPDATE ------------------------
FTPRequest.EnableSsl = True
'----------------------- UPDATE ------------------------
Using FtpResponse As WebResponse = FTPRequest.GetResponse,
DirListStream As Stream = FtpResponse.GetResponseStream(),
listReader As StreamReader = New StreamReader(DirListStream)
While Not listReader.EndOfStream
Dim DirContent As String = listReader.ReadLine()
If DirContent.Contains(Path.GetFileNameWithoutExtension(BGWorkerObj.ResourceURI)) Then
BGWorkerObj.FileLength = Convert.ToInt64(DirContent.Split(New String() {" "}, StringSplitOptions.RemoveEmptyEntries)(4))
BGW.ReportProgress(0, BGWorkerObj)
Exit While
End If
End While
End Using
'----------------------- END UPDATE ------------------------
'Using FtpResponse As WebResponse = FTPRequest.GetResponse
' BGWorkerObj.FileLength = Convert.ToInt64(FtpResponse.ContentLength)
' BGW.ReportProgress(0, BGWorkerObj)
'End Using
If BGW.CancellationPending Then e.Cancel = True
Try
FTPRequest = CType(WebRequest.Create(BGWorkerObj.ResourceURI), FtpWebRequest)
FTPRequest.EnableSsl = True
FTPRequest.Credentials = New NetworkCredential(BGWorkerObj.UserName, BGWorkerObj.Password)
FTPRequest.Method = WebRequestMethods.Ftp.DownloadFile
Using Response As FtpWebResponse = DirectCast(FTPRequest.GetResponse, FtpWebResponse)
If Response.StatusCode > 299 Then
e.Result = 0
Throw New Exception("The Ftp Server rejected the request. StatusCode: " &
Response.StatusCode.ToString(),
New InvalidOperationException(Response.StatusCode.ToString()))
Exit Sub
End If
Using stream = Response.GetResponseStream(),
fileStream As FileStream = File.Create(BGWorkerObj.FilePath)
Dim read As Integer
Dim buffer As Byte() = New Byte(BufferSize - 1) {}
Do
read = stream.Read(buffer, 0, buffer.Length)
fileStream.Write(buffer, 0, read)
BGWorkerObj.DownloadedBytes += read
BGWorkerObj.BytesToDownload = BGWorkerObj.FileLength - BGWorkerObj.DownloadedBytes
If BGW.CancellationPending Then
e.Cancel = True
Exit Do
Else
BGW.ReportProgress(CInt((CSng(BGWorkerObj.DownloadedBytes) / BGWorkerObj.FileLength) * 100), BGWorkerObj)
End If
Loop While read > 0
End Using
End Using
Catch ex As Exception
If e.Cancel = False Then Throw
Finally
If e.Cancel = True Then
If File.Exists(BGWorkerObj.FilePath) Then
File.Delete(BGWorkerObj.FilePath)
End If
End If
End Try
End Sub
Private Sub BWorkerD_ProgressChanged(sender As Object, e As ProgressChangedEventArgs)
pBarD.Value = e.ProgressPercentage
lblPercentD.Text = e.ProgressPercentage.ToString() & " %"
If lblFileSizeD.Text.Length = 0 Then
lblFileSizeD.Text = CType(e.UserState, BGWorkerObject).FileLength.ToString("N0", CultureInfo.CurrentUICulture.NumberFormat)
End If
lblDownloadedBytesD.Text = CType(e.UserState, BGWorkerObject).DownloadedBytes.ToString("N0", CultureInfo.CurrentUICulture.NumberFormat)
If e.ProgressPercentage <= 15 Then
lblDownloadedBytesD.ForeColor = Color.Red
ElseIf e.ProgressPercentage <= 66 Then
lblDownloadedBytesD.ForeColor = Color.Orange
Else
lblDownloadedBytesD.ForeColor = Color.LightGreen
End If
End Sub
Private Sub BWorkerD_RunWorkerCompleted(sender As Object, e As RunWorkerCompletedEventArgs)
Dim DownloadAborted As Boolean = False
If e.Error IsNot Nothing Then
DownloadAborted = True
lblDownloadedBytesD.ForeColor = Color.Red
lblDownloadedBytesD.Text = "Error!"
ElseIf e.Cancelled Then
DownloadAborted = True
lblDownloadedBytesD.ForeColor = Color.Yellow
lblDownloadedBytesD.Text = "Cancelled!"
pBarD.Value = 0
lblPercentD.Text = "0%"
Else
lblDownloadedBytesD.ForeColor = Color.LightGreen
lblDownloadedBytesD.Text = "Download completed"
End If
End Sub
Private Sub btnAbortDownload_Click(sender As Object, e As EventArgs) Handles btnAbortDownload.Click
BWorkerD.CancelAsync()
End Sub
End Class
A visual result of the operation described:
