Multithreading A Function in VB.Net - vb.net

Question

I am trying to multi thread my application so as it is visible while it is executing the process, this is what I have so far:
Private Sub SendPOST(ByVal URL As String)
Try
Dim DataBytes As Byte() = Encoding.ASCII.GetBytes("")
Dim Request As HttpWebRequest = TryCast(WebRequest.Create(URL.Trim & "/webdav/"), HttpWebRequest)
Request.Method = "POST"
Request.ContentType = "application/x-www-form-urlencoded"
Request.ContentLength = DataBytes.Length
Request.Timeout = 1000
Request.ReadWriteTimeout = 1000
Dim PostData As Stream = Request.GetRequestStream()
PostData.Write(DataBytes, 0, DataBytes.Length)
Dim Response As WebResponse = Request.GetResponse()
Dim ResponseStream As Stream = Response.GetResponseStream()
Dim StreamReader As New IO.StreamReader(ResponseStream)
Dim Text As String = StreamReader.ReadToEnd()
PostData.Close()
Catch ex As Exception
If ex.ToString.Contains("401") Then
TextBox2.Text = TextBox2.Text & URL & "/webdav/" & vbNewLine
End If
End Try
End Sub
Public Sub G0()
Dim siteSplit() As String = TextBox1.Text.Split(vbNewLine)
For i = 0 To siteSplit.Count - 1
Try
If siteSplit(i).Contains("http://") Then
SendPOST(siteSplit(i).Trim)
Else
SendPOST("http://" & siteSplit(i).Trim)
End If
Catch ex As Exception
End Try
Next
End Sub
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim t As Thread
t = New Thread(AddressOf Me.G0)
t.Start()
End Sub
However, the 'G0' sub code is not being executed at all, and I need to multi thread the 'SendPOST' as that is what slows the application.

Answer 1

Catch ex As Exception
End Try
A very effective way to stop .NET from telling you what you did wrong. Not knowing why it doesn't work is however the inevitable outcome.
Delete that.

Answer 2

Public Class Form1
'This just shows some concepts of threading.
'it isn't intended to do anything
'requires a Button, and two Labels
'
Private Sub Button1_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) _
Handles Button1.Click
'starts / stops a test thread
'isRun = 0 no thread running, start one
'isRun = 1 thread running, stop it
If Threading.Interlocked.Read(isRun) = 0L Then
'start thread
Threading.Interlocked.Increment(isRun)
t = New Threading.Thread(AddressOf showTime)
'simple threading app - display time about twice per second
t.IsBackground = True 'from a background thread
t.Start()
Else
'stop thread
Threading.Interlocked.Exchange(isRun, 0L)
t.Join() 'wait for thread to end
Threading.Monitor.Enter(listLock)
intervalList.Clear() 'clear the list
Threading.Monitor.Exit(listLock)
Label1.Text = "Stop"
Label2.Text = ""
End If
End Sub
Dim t As Threading.Thread
Dim intervalList As New List(Of Double)
Dim listLock As New Object
Dim isRun As Long = 0L
Private Sub showTime()
Dim dlgt As New UpdLblDel(AddressOf UpdateLabel) 'delegate for UI access
Dim lastDateTime As DateTime = Nothing
Do
Dim d As DateTime = DateTime.Now
If lastDateTime <> Nothing Then
'record difference of times - check sleep interval
Threading.Monitor.Enter(listLock)
intervalList.Add((d - lastDateTime).TotalMilliseconds)
Threading.Monitor.Exit(listLock)
End If
lastDateTime = DateTime.Now
dlgt.BeginInvoke(d, Nothing, Nothing) 'update the UI - note immediate return
Threading.Thread.Sleep(500) 'sleep for approx. 500 ms.
Loop While Threading.Interlocked.Read(isRun) = 1L
End Sub
Delegate Sub UpdLblDel(ByVal theTime As Object)
Private Sub UpdateLabel(ByVal theTime As Object)
If Threading.Interlocked.Read(isRun) = 1L Then
If Label1.InvokeRequired Then 'prevent cross-thread errors
Label1.BeginInvoke(New UpdLblDel(AddressOf UpdateLabel), theTime)
Exit Sub
Else
Label1.Text = CType(theTime, DateTime).ToString("HH:mm:ss.f") 'show the time from the background thread
End If
If Threading.Interlocked.Read(intervalList.Count) >= 10L Then
'take average
Threading.Monitor.Enter(listLock)
Dim avg As Double = intervalList.Sum / intervalList.Count 'sum all of the intervals / count
intervalList.Clear() 'clear the list
intervalList.Add(avg) 'forward the average
Label2.Text = avg.ToString("n2") 'show average
Threading.Monitor.Exit(listLock)
End If
End If
End Sub
End Class

Answer 3

You have to wrap the method that accesses the UI component in a delegate (it doesn't have to be a named delegate; it can be anonymous or an Action or Func), and then pass that to Me.Invoke, as others have alluded to.
In this example, I'm wrapping the split functionality in a lambda, and assigning that lambda to a variable of type Func(Of String()). I then pass that variable to Me.Invoke.
Public Sub G0()
Dim siteSplitFunc As Func(Of String()) = Function() _
TextBox1.Text.Split(vbNewLine.ToCharArray())
Dim siteSplit As String() = CType(Me.Invoke(siteSplitFunc), String())
For i = 0 To siteSplit.Count - 1
Try
If siteSplit(i).Contains("http://") Then
SendPOST(siteSplit(i).Trim)
Else
SendPOST("http://" & siteSplit(i).Trim)
End If
Catch ex As Exception
'Do something useful
End Try
Next
End Sub

Answer 4

You cannot access UI object directly from a thread.
When you want to read/write a textbox, you have to do it in the UI thread. This can be done by using Invoke. Or better yet, send/receive the information with parameters.

Answer 5

Here's a Delegate and a matching method. You call the method to update the textbox and it figures out if it should proxy the method for you by basically asking form if its on the same thread:
Private Delegate Sub UpdateTextBoxDelegate(ByVal text As String)
Private Sub UpdateTextBox(ByVal text As String)
If Me.InvokeRequired Then
Me.Invoke(New UpdateTextBoxDelegate(AddressOf UpdateTextBox), text)
Else
TextBox2.Text &= text
End If
End Sub
To use it, just change your catch statement to:
If ex.ToString.Contains("401") Then
UpdateTextBox(URL & "/webdav/" & vbNewLine)
End If