VB.Net Multiple background workers - Only last task completes - vb.net

Question

I have been pulling my hair out trying to get this to work. If I step through the code in debugger it all works great.
My problem is if I just run it, only the last task responds. I'm guessing I am overwriting the background working or something. I am sure I am doing a few things wrong but my code is now messy as I tried many way while searching. I know of the threadpool and .Net 4.0 tasks but having a hard time getting to do what I need.
Basicly I am writing a program (trying more likely) that takes a list of computers and pings then, then checks their uptime and reports back.
This works fine in the UI thread (Obviously that locks up my screen). I can have the background worker just do this, but then it does each computer 1 by one, and while the screen is responsive it still takes a long time.
So my answer was to have a for loop for each server launching a new background worker thread. My solution does not work.
I have seen other threads that I could do it, but I need to use with events to call code to update to UI when each is done.
What is the most simple way to do this?
Here is my code. Most is just copy paste + modify till I get it working right.
So In the main class I have the testworker.
(I tried using Testworker() but it said I could not do that WithEvents)
When I click the button the list loads.
Private WithEvents TestWorker As System.ComponentModel.BackgroundWorker
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs) Handles Button1.Click
Button1.IsEnabled = False
Dim indexMax As Integer
indexMax = DataGridStatus.Items.Count
For index = 1 To (indexMax)
Dim Temp As ServerInfo = DataGridStatus.Items(index - 1)
Temp.Index = index - 1
Call_Thread(Temp)
Next
End Sub
Private Sub Call_Thread(ByVal server As ServerInfo)
Dim localserver As ServerInfo = server
TestWorker = New System.ComponentModel.BackgroundWorker
TestWorker.WorkerReportsProgress = True
TestWorker.WorkerSupportsCancellation = True
TestWorker.RunWorkerAsync(localserver)
End Sub
Private Sub TestWorker_DoWork(ByVal sender As Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles TestWorker.DoWork
Dim iparray As IPHostEntry
Dim ip() As IPAddress
Dim Server As ServerInfo
Server = e.Argument
Try
'Get IP Address first
iparray = Dns.GetHostEntry(Server.ServerName)
ip = iparray.AddressList
Server.IPAddress = ip(0).ToString
'Try Pinging
Server.PingResult = PingHost(Server.ServerName)
If Server.PingResult = "Success" Then
'If ping success, get uptime
Server.UpTime = GetUptime(Server.ServerName)
Else
Server.PingResult = "Failed"
End If
Catch ex As Exception
Server.PingResult = "Error"
End Try
TestWorker.ReportProgress(0, Server)
Thread.Sleep(1000)
End Sub
Private Sub TestWorker_ProgressChanged(ByVal sender As Object, ByVal e As System.ComponentModel.ProgressChangedEventArgs) Handles TestWorker.ProgressChanged
Dim index As Integer
Dim serverchange As ServerInfo = DirectCast(e.UserState, ServerInfo)
index = DataGridStatus.Items.IndexOf(serverchange)
' index = serverchange.Index
DataGridStatus.Items.Item(index) = serverchange
' ProgressBar1.Value = e.ProgressPercentage
DataGridStatus.Items.Refresh()
End Sub

Answer 1

You are only getting the last result because you are blowing away your BackgroundWorker each time you call TestWorker = New System.ComponentModel.BackgroundWorker. Since the processing is being done asynchronously, this line is being called multiple times within your for loop before the previous work has finished.
Something like the following might work. (Sorry, my VB is rusty; there are probably more efficient ways of expressing this.)
Delegate Function PingDelegate(ByVal server As String) As String
Private _completedCount As Int32
Private ReadOnly _lockObject As New System.Object
Dim _rnd As New Random
Private _servers As List(Of String)
Private Sub GoButton_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles GoButton.Click
_servers = New List(Of System.String)(New String() {"adam", "betty", "clyde", "danny", "evan", "fred", "gertrude", "hank", "ice-t", "joshua"})
_completedCount = 0
ListBox1.Items.Clear()
GoButton.Enabled = False
BackgroundWorker1.RunWorkerAsync(_servers)
End Sub
Private Sub BackgroundWorker1_DoWork(ByVal sender As System.Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
Dim servers As List(Of System.String) = DirectCast(e.Argument, List(Of System.String))
Dim waitHandles As New List(Of WaitHandle)
For Each server As System.String In servers
' Get a delegate for the ping operation. .Net will let you call it asynchronously
Dim d As New PingDelegate(AddressOf Ping)
' Start the ping operation async. When the ping is complete, it will automatically call PingIsDone
Dim ar As IAsyncResult = d.BeginInvoke(server, AddressOf PingIsDone, d)
' Add the IAsyncResult for this invocation to our collection.
waitHandles.Add(ar.AsyncWaitHandle)
Next
' Wait until everything is done. This will not block the UI thread because it is happening
' in the background. You could also use the overload that takes a timeout value and
' check to see if the user has requested cancellation, for example. Once all operations
' are complete, this method will exit scope and the BackgroundWorker1_RunWorkerCompleted
' will be called.
WaitHandle.WaitAll(waitHandles.ToArray())
End Sub
Private Sub BackgroundWorker1_ProgressChanged(ByVal sender As System.Object, ByVal e As System.ComponentModel.ProgressChangedEventArgs) Handles BackgroundWorker1.ProgressChanged
ListBox1.Items.Add(String.Format("{0} ({1}% done)", e.UserState, e.ProgressPercentage))
End Sub
Private Sub BackgroundWorker1_RunWorkerCompleted(ByVal sender As System.Object, ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles BackgroundWorker1.RunWorkerCompleted
GoButton.Enabled = True
End Sub
Private Function Ping(ByVal server As System.String) As System.String
' Simulate a ping with random result and duration
Threading.Thread.Sleep(_rnd.Next(1000, 4000))
Dim result As Int32 = _rnd.Next(0, 2)
If result = 0 Then
Return server & " is ok"
Else
Return server & " is down"
End If
End Function
Private Sub PingIsDone(ByVal ar As IAsyncResult)
' This method is called everytime a ping operation completes. Note that the order in which
' this method fires is completely independant of the order of the servers. The first server
' to respond calls this method first, etc. This keeps optimal performance.
Dim d As PingDelegate = DirectCast(ar.AsyncState, PingDelegate)
' Complete the operation and get the result.
Dim pingResult As String = d.EndInvoke(ar)
' To be safe, we put a lock around this so that _completedCount gets incremented atomically
' with the progress report. This may or may not be necessary in your application.
SyncLock (_lockObject)
_completedCount = _completedCount + 1
Dim percent As Int32 = _completedCount * 100 / _servers.Count
BackgroundWorker1.ReportProgress(percent, pingResult)
End SyncLock
End Sub

Answer 2

Update: I posted this answer focusing on exactly what you were trying to do from a technical standpoint (use many background workers) without really putting much thought into whether or not this was a good way to accomplish your real objective. In fact, I think you could achieve what you're going for much more easily with a single BackgroundWorker and something like a Parallel.ForEach loop in its DoWork event handler (this takes care of a lot of the nitty gritty work in, e.g., Dave's solution).
When you declare WithEvents TestWorker As BackgroundWorker in VB it wraps it up something like this (not exactly—this is just to illustrate the idea):
Private _TestWorker As BackgroundWorker
Private Property TestWorker As BackgroundWorker
Get
Return _TestWorker
End Get
Set(ByVal value As BackgroundWorker)
' This is all probably handled in a more thread-safe way, mind you. '
Dim prevWorker As BackgroundWorker = _TestWorker
If prevWorker IsNot Nothing Then
RemoveHandler prevWorker.DoWork, AddressOf TestWorker_DoWork
' etc. '
End If
If value IsNot Nothing Then
AddHandler value.DoWork, AddressOf TestWorker_DoWork
' etc. '
End If
_TestWorker = value
End Set
End Property
When you realize this, it becomes clear that by setting TestWorker to a new BackgroundWorker on every call to Call_Thread, you are removing any attached handlers from the object previously referenced by the field.
The most obvious fix would simply be to create a new local BackgroundWorker object in each call to Call_Thread, attach the handlers there (using AddHandler and RemoveHandler), and then just let it do its thing:
Private Sub Call_Thread(ByVal server As ServerInfo)
Dim localserver As ServerInfo = server
' Use a local variable for the new worker. '
' This takes the place of the Private WithEvents field. '
Dim worker As New System.ComponentModel.BackgroundWorker
' Set it up. '
With worker
.WorkerReportsProgress = True
.WorkerSupportsCancellation = True
End With
' Attach the handlers. '
AddHandler worker.DoWork, AddressOf TestWorker_DoWork
AddHandler worker.ProgressChanged, AdressOf TestWorker_ProgressChanged
' Do the work. '
worker.RunWorkerAsync(localserver)
End Sub
Creating the worker right there in the method should be fine as long as you do so from the UI thread, since BackgroundWorker automatically attaches to the current SynchronizationContext in its constructor (if I remember correctly).

Answer 3

Ideally you should use only 1 backgroundworker and use it like this:
Assemble all the work that needs to be done: in your case a list of ServerInfo
Do the work in the background: ping all the servers and keep the result
Report progress: for example after each server pinged
Put results back in DoWorkEventArgs.Result
Display the results back in your UI.

Answer 4

You need to attach TestWorker_DoWork and TestWorker_ProgressChanged to the DoWork and ProgressChanged events within Call_Thread. I haven't yet examined the rest of the code, but that is why it isn't doing anything now.
TestWorker = New System.ComponentModel.BackgroundWorker
TestWorker.WorkerReportsProgress = True
TestWorker.WorkerSupportsCancellation = True
AddHandler TestWorker.DoWork, AddressOf TestWorker_DoWork
AddHandler TestWorker.ProgressChanged, AddressOf TestWorker_ProgressChanged
TestWorker.RunWorkerAsync(localserver)