CODEWITHSUNDEEP
vb.netmultithreading
Peterp
Peterp

Reputation: 100

Problems with Threading in vb.net

Background: I have a program that is processing lots of database records, and generating tasks to do. (In this case creating user accounts in AD). Part of this is to create the user directories, for profiles and home directories, and setting the permissions on them. This needs to wait until the ad account has replicated across all of our DC's.

So, my program will have a separate thread responsible for creating the directories, that will process a queue populated from the main thread.

I've done some research on Threading and come up with the following code pattern:

Imports System.Threading

Public Class Form1
Dim worker As Object
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    worker = New workerObj(AddressOf resultcallback)
    Dim t As New Thread(AddressOf worker.mainloop)
End Sub

Public Sub resultcallback(ByVal item As String)
    Outbox.AppendText(item)
End Sub

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    worker.addItem(inbox.Text)
End Sub
End Class

Public Delegate Sub resultcallback(ByVal item As String)

Public Class workerObj
Private myQueue As New Queue(Of String)
Private myCallback As resultcallback
Dim item As String = "nout"

Public Sub New(ByVal callbackdelegate As resultcallback)
    myCallback = callbackdelegate
End Sub

Public Sub mainloop()
    While True
        If myQueue.Count > 0 Then
            item = myQueue.Dequeue()
            myCallBack(item)
        End If
        Thread.Sleep(5000)
    End While
End Sub

Public Sub addItem(ByVal item As String)
    myQueue.Enqueue(item)
End Sub
End Class

Problem: On the line Dim t as new Thread.....

Error   1   Overload resolution failed because no accessible 'New' is most specific for these arguments:
'Public Sub New(start As System.Threading.ParameterizedThreadStart)': Not most specific.
'Public Sub New(start As System.Threading.ThreadStart)': Not most specific. n:\visual studio 2013\Projects\ThreadTest\ThreadTest\Form1.vb   7   13  ThreadTest

Can anyone help tell me where I have gone wrong? Cheers.

Upvotes: 0

Views: 1337

Answers (1)

Panagiotis Kanavos
Panagiotis Kanavos

Reputation: 131189

Threads do not have a public constructor, you need to call Thread.Start. I'd suggest you don't do that though. Writing thread-safe code is tricky enough when you do know about multithreaded programming.

Eg in your code you modify a Queue from two different threads without locking. Queue isn't thread safe and you can corrupt the queue. You should lock access to it or use ConcurrentQueue which is thread-safe. Another error is trying to modify a TextBox from another thread - this will lead to an Exception because only the UI thread is allowed to modify UI controls.

A better option though is to use the ActionBlock class from the DataFlow library which already does what you want: queue requests and process them in one or more separate threads.

Your code can be as simple as this:

Dim myFileWorker=New ActionBlock(Of string)(Function(path) =>DoSomething(path))

For Each somePath As String in ListWithManyPaths
    myFileWorker.Post(somePath)
Next somePath

myFileWorker.Complete()
myFileWorker.Completion.Wait()

By default only one path will be processed at a time. To process multiple paths you pass an ExecutionDataflowBlockOptions object with the desired MaxDegreeOfParallelism:

Dim options=New ExecutionDataflowBlockOptions() With { .MaxDegreeOfParallelism=5}
Dim myFileWorker=New ActionBlock(Of String) Function(path) DoSomething(path),options)

Upvotes: 2

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