How to track multiple BackgroundworkerX.Runworkercompleted operations - vb.net

Question

I am trying to use a single handler to cover the end of multiple backgroundworker activities and cannot find a way to get the information about the specific backgroundworker using the backgroundworkercompleted event.
My code to catch the event is as below:
Private Sub BGx_RunWorkerCompleted(ByVal sender As Object, ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles BackgroundWorker1.RunWorkerCompleted, BackgroundWorker2.RunWorkerCompleted, BackgroundWorker3.RunWorkerCompleted, BackgroundWorker4.RunWorkerCompleted, BackgroundWorker5.RunWorkerCompleted, BackgroundWorker6.RunWorkerCompleted, BackgroundWorker7.RunWorkerCompleted, BackgroundWorker8.RunWorkerCompleted
'Do work here based on completed Backgroundworker
For BG = 1 To 8
If Not DSWorkers(BG).IsBusy Then
If DStatus(BG) = -2 Then : DStatus(BG) = -1 : End If
End If
Next
Complete()
End Sub
There is nothing on the "Do Work Here" section because I do not know how to capture and have been unable to find details of the backgroundworkercompleted event id.
Please - any pointers as to how I can identify the specific completed BackgroundWorker

Answer 1

As with all event handlers, the sender parameter is a reference to the object that raised the event, so you can access the actual BackgroundWorker that has completed its work via that. If you need some data other than that, you assign it to the e.Result property in the DoWork event handler and get it back from the e.Result property in the RunWorkerCompleted event handler. e.Result works for getting data out of the DoWork event handler much as e.Argument works for getting data in.
Check this out for some examples of using BackgroundWorker objects, including passing data out using e.Result. You might also like to checkout my own BackgroundMultiWorker class, which basically combines the functionality of multiple BackgroundWorker objects into a single BackgroundMultiWorker object. It identifies each task using a token.
EDIT:
Here's an example that may help with this issue and your task in general:
Imports System.ComponentModel
Imports System.Threading
Public Class Form1
Private ReadOnly resultsByWorker As New Dictionary(Of BackgroundWorker, BackgroundWorkerResult)
Private ReadOnly rng As New Random
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
'The NumericUpDown is used to select the index of a BackgroundWorker to cancel.
With NumericUpDown1
.DecimalPlaces = 0
.Minimum = 0
.Maximum = 9
End With
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
'Create 10 BackgroundWorkers and run them.
For i = 1 To 10
Dim worker As New BackgroundWorker
resultsByWorker.Add(worker, New BackgroundWorkerResult)
AddHandler worker.DoWork, AddressOf workers_DoWork
AddHandler worker.RunWorkerCompleted, AddressOf workers_RunWorkerCompleted
worker.WorkerSupportsCancellation = True
worker.RunWorkerAsync()
Next
End Sub
Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
Dim index = Convert.ToInt32(NumericUpDown1.Value)
Dim worker = resultsByWorker.Keys.ToArray()(index)
If worker.IsBusy Then
'Cancel the BackgroundWorker at the specified index.
worker.CancelAsync()
End If
End Sub
Private Sub workers_DoWork(sender As Object, e As DoWorkEventArgs)
Dim worker = DirectCast(sender, BackgroundWorker)
'Do work for a random number of seconds between 10 and 20.
Dim period = rng.Next(10, 20 + 1)
For i = 0 To period
If worker.CancellationPending Then
e.Cancel = True
Return
End If
'Simulate work.
Thread.Sleep(1000)
Next
'The work was completed without being cancelled.
e.Result = period
End Sub
Private Sub workers_RunWorkerCompleted(sender As Object, e As RunWorkerCompletedEventArgs)
Dim worker = DirectCast(sender, BackgroundWorker)
Dim result = resultsByWorker(worker)
If e.Cancelled Then
result.WasCancelled = True
Else
result.Result = CInt(e.Result)
End If
Dim workers = resultsByWorker.Keys.ToArray()
If Not workers.Any(Function(bgw) bgw.IsBusy) Then
'All work has completed so display the results.
Dim results As New List(Of String)
For i = 0 To workers.GetUpperBound(0)
worker = workers(i)
result = resultsByWorker(worker)
results.Add($"Worker {i} {If(result.WasCancelled, "was cancelled", $"completed {result.Result} iterations")}.")
Next
MessageBox.Show(String.Join(Environment.NewLine, results))
End If
End Sub
End Class
Public Class BackgroundWorkerResult
Public Property WasCancelled As Boolean
Public Property Result As Integer
End Class
Here is that example reworked to use a single instance of the BackgroundMultiWorker is linked to instead of multiple instances of the BackgroundWorker class.
Imports System.Threading
Public Class Form1
Private WithEvents worker As New BackgroundMultiWorker With {.WorkerSupportsCancellation = True}
Private ReadOnly results(9) As BackgroundWorkerResult
Private ReadOnly rng As New Random
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
'The NumericUpDown is used to select the index of a BackgroundWorker to cancel.
With NumericUpDown1
.DecimalPlaces = 0
.Minimum = 0
.Maximum = results.GetUpperBound(0)
End With
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
'Create 10 BackgroundWorkers and run them.
For i = 0 To results.GetUpperBound(0)
results(i) = New BackgroundWorkerResult
worker.RunWorkerAsync(i)
Next
End Sub
Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
Dim index = Convert.ToInt32(NumericUpDown1.Value)
If worker.IsBusy(index) Then
'Cancel the BackgroundWorker at the specified index.
worker.CancelAsync(index)
End If
End Sub
Private Sub worker_DoWork(sender As Object, e As DoWorkEventArgs) Handles worker.DoWork
'Do work for a random number of seconds between 10 and 20.
Dim period = rng.Next(10, 20 + 1)
For i = 0 To period
If worker.IsCancellationPending(e.Token) Then
e.Cancel = True
Return
End If
'Simulate work.
Thread.Sleep(1000)
Next
'The work was completed without being cancelled.
e.Result = period
End Sub
Private Sub workers_RunWorkerCompleted(sender As Object, e As RunWorkerCompletedEventArgs) Handles worker.RunWorkerCompleted
Dim result = results(CInt(e.Token))
If e.Cancelled Then
result.WasCancelled = True
Else
result.Result = CInt(e.Result)
End If
If Not worker.IsBusy() Then
'All work has completed so display the results.
Dim output As New List(Of String)
For i = 0 To results.GetUpperBound(0)
result = results(i)
output.Add($"Task {i} {If(result.WasCancelled, "was cancelled", $"completed {result.Result} iterations")}.")
Next
MessageBox.Show(String.Join(Environment.NewLine, output))
End If
End Sub
End Class
Public Class BackgroundWorkerResult
Public Property WasCancelled As Boolean
Public Property Result As Integer
End Class