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There have been several solutions to multithreading and creating responsive user interfaces. Tell us some of the methods you have found in VB6 or your experiences with multithreading in VB.NET.
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Add Multithreading to Your VB.NET ApplicationsAverage Rating: 4.4/5 | Rate this item | 128 users have rated this item.
	

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Add Multithreading to Your VB.NET Applications (cont'd)
Passing Data Through Multithreaded Procedures
The last example shows a rather simple situation. Multithreading has many complications that you have to work out when you program. One issue that you will run into is passing data to and from the procedure passed to the constructor of the Thread class. That is to say, the procedure you want to kick off on another thread cannot be passed any parameters and you cannot return data from that procedure. This is because the procedure you pass to the thread constructor cannot have any parameters or return value. To get around this, wrap your procedure in a class where the parameters to the method are written as fields of the class.
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A simple example of this would be if we had a procedure that calculated the square of a number:

Function Square(ByVal Value As Double) As Double
    Return Value * Value
End Function

To make this procedure available to be used in a new thread we would wrap it in a class:

Public Class SquareClass
    Public Value As Double
    Public Square As Double

    Public Sub CalcSquare()
        Square = Value * Value
    End Sub
End Class

Use this code to start the CalcSquare procedure on a new thread. following code:

Private Sub Button1_Click(ByVal sender As System.Object, _
        ByVal e As System.EventArgs) Handles Button1.Click

    Dim oSquare As New SquareClass()

    t = New Thread(AddressOf oSquare.CalcSquare)

    oSquare.Value = 30

    t.Start()
End Sub

Notice that after the thread is started, we do not inspect the square value of the class, because it is not guaranteed to have executed once you call the start method of the thread. There are a few ways to retrieve values back from another thread. The easiest way is to raise an event when the thread is complete. We will examine another method in the next section on thread synchronization. The following code adds the event declarations to the SquareClass.

Public Class SquareClass
    Public Value As Double
    Public Square As Double

    Public Event ThreadComplete(ByVal Square As Double)

    Public Sub CalcSquare()
        Square = Value * Value
        RaiseEvent ThreadComplete(Square)
    End Sub
End Class

Catching the events in the calling code has not changed much from VB6, you still declare the variables WithEvents and handle the event in a procedure. The part that has changed is that you declare that a procedure handles the event using the Handles keyword and not through the naming convention of Object_Event as in VB6.

Dim WithEvents oSquare As SquareClass

Private Sub Button1_Click(ByVal sender As System.Object, _
        ByVal e As System.EventArgs) Handles Button1.Click

    oSquare = New SquareClass()

    t = New Thread(AddressOf oSquare.CalcSquare)

    oSquare.Value = 30
    t.Start()
End Sub

Sub SquareEventHandler(ByVal Square As Double) _
        Handles oSquare.ThreadComplete

    MsgBox("The square is " & Square)

End Sub

The one thing to note with this method is that the procedure handling the event, in this case SquareEventHandler, will run within the thread that raised the event. It does not run within the thread from which the form is executing.



Previous Page: Keep It Under Control 	Next Page: Synchronizing the Threads
Page 1: Working with Threads	Page 3: Passing Data Through Multithreaded Procedures
Page 2: Keep It Under Control	Page 4: Synchronizing the Threads
	

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Please rate this item (5=best)
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By dghervas     March 13 2010 8:07 PM PDT

i want to disable or enable a control that doesent suport invoke method (from a diferent thread offcourse) how would i do that? A timer to be more specific. If i do it as i would normaly do "timer1.enabled = true" it doesent give any error but doesent work either.
I would apreciate some help, thanks

Reply to this comment
By erwinyn@gmail.com     January 9 2010 6:30 PM PDT

End Class

Class ThreadProc
Delegate Sub UpdateTextCallBack(ByVal ctrl As TextBox, ByVal msg As Int32)
Dim ctrl As TextBox
Dim msg As Int32
Public t As Thread

' create 3 thread for handle 3 counter
Public Sub StartCounter(ByVal ctrl As TextBox, ByVal msg As Int32)
Me.ctrl = ctrl: Me.msg = msg
t = New Thread(AddressOf Counter)
t.Start()
End Sub

' counter procedure
Public Sub Counter()
Try
While True
UpdateTextControl(ctrl, msg)
msg = msg 1
Thread.Sleep(100) ' delay counter in mili second
If msg > 100 Then msg = 0
End While
Catch at As ThreadAbortException
MessageBox.Show(at.Message) ' message: thread was being aborted
End Try
End Sub

' update TextBox value used Invoke metode (thread-safe)
Public Sub UpdateTextControl(ByVal ctrl As TextBox, ByVal msg As Int32)
If ctrl.InvokeRequired Then
ctrl.Invoke(New UpdateTextCallBack(AddressOf UpdateTextControl), New Object() {ctrl, msg})
Else
ctrl.Text = msg.ToString
End if
End Sub
End Class
' ..FINISH

Reply to this comment
By erwinyn     January 9 2010 6:23 PM PDT

Imports System
Imports System.Net
Imports System.Windows
Imports System.Threading

' add 3 TextBox and 2 Button on your form then run this sample (used vb2008)

Public Class Form1

' for save object in array list
Private threadList As ArrayList = ArrayList.Synchronized(New ArrayList)

' running 3 counter with 3 thread
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim out() As TextBox = {TextBox1, TextBox2, TextBox3}
For i As Int32 = 0 To 2
Dim obj As New ThreadProc
obj.StartCounter(out(i), 0)
threadList.Add(obj)
Next
End Sub

' stop all counter
Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
For i As Int32 = 0 To threadList.Count - 1
Dim x As ThreadProc = TryCast(threadList(i), ThreadProc)
If x IsNot Nothing Then
If x.t.ThreadState ThreadState.Aborted Then
x.t.Abort()
End If
End If
Next
End Sub

' (continue)

Reply to this comment
By LyndonL     January 7 2010 6:59 PM PDT

It may be worth mentioning that that sub BackgroundProcess will be created in a new thread, which differs from the thread that Form1 (where the Listbox resides) was created. you cannot directly manipulate items created in different threads.
Workaround:
Use Delegate/Invoke method:

After Form1 Class insert:
Delegate Sub listbox1Delegate(ByVal str As String)

At bottom of class insert:
Private Sub Setlistbox1(ByVal str As String)
If listbox1.InvokeRequired Then
listbox1.Invoke(New listbox1Delegate(AddressOf Setlistbox1), str)
Else
listbox1.items.add = str
End If
End Sub

Then, inplace of the ListBox1.Items.Add("Iterations: " i) in the BackgroundProcess sub, change it to:
setlistbox1("Iterations: " i)

Reply to this comment
By tqnga     December 15 2009 3:48 AM PDT

Hello I have a problem when using Thread:
Public Class ConvertThread
Public SourceTable as DataTable
Public iRowCount as integer=0
Public Sub PerformConvert()
SyncLock GetType(ConvertToUnicode)
For Each oRow As DataRow In SourceTable.Rows
iRowCount = 1
'I want thread'll be stopped when iRowCount=SourceTable.Rows.Count
Next
End SyncLock
End Class
Call thread:
For Each oTable As DataTable In sourceDataset.Tables
Dim oConvert As New ConvertThread
Dim oThread As New Threading.Thread(AddressOf oConvert.PerformConvert)
oConvert .SourceTable = oTable
oThread.Start()
If oConvert.iRowCount = oTable.Rows.Count Then
oThread.Abort()
oTable.AcceptChanges()
End If
Next
But I get exception "Collection was modified; enumeration operation may not execute." when using this code. And I think this code don't help me manage threads which I declare.
Pls help me. Thank you very much!


Reply to this comment
By Ramana     December 6 2009 1:57 AM PDT

I'm created one service, check by below code.
Threading not working properly,
Protected Overrides Sub OnStart(ByVal args() As String)
Try
t = New System.Timers.Timer(TimerInterval) '10 secs
AddHandler t.Elapsed, AddressOf TimerFired
With t
.AutoReset = True
.Enabled = True
.Start()
End With
Catch ex As Exception
writeErrorToFile(ErrorLogFileName, ex.ToString)
End Try
End Sub
Private Sub TimerFired(ByVal sender As Object, ByVal e As ElapsedEventArgs)
Try
'currently there are 3 processes
CheckQueueToNotify() 'to display desktop notification
CheckQueueToProcess("Local") ' to process queues in local database
CheckQueueToProcessServer() ' to process queues in server database
Catch ex As Exception
writeErrorToFile(ErrorLogFileName, ex.ToString)
End Try
End Sub
The three functions not running properly, when i'm debug the focus goto first function and suddenly focus move off and run second function. totally weired.
pls giv me suggestion.

Reply to this comment
By dusky     September 28 2009 3:49 PM PDT

thanks for creating a easy to understand guide ^^

Most other guides ive read on multi threading are all over the place and i couldnt figure out how to do it.

Got it first time with this 1 tho =)

Reply to this comment
By Steve     September 21 2009 4:16 AM PDT

When I posted my previous comments I had only read the first page of this article. On reading the rest I realize that the whole article is very bad. It presents examples that just won't work and shows that the author does not really understand multi-threading of Windows Forms.

You can not safely call a Windows Forms object (a control) from within a thread that did not create it. Any attempts to do so at best won't work and at worst may corrupt data within your program.

My suggested solution is the correct way to proceed, but this article should be removed or re-written because its advice is very misleading.

Reply to this comment
By Jasbir Singh     September 7 2009 3:07 AM PDT

when i run the above code, i got this error,

"Cross-thread operation not valid: Control 'ListBox1' accessed from a thread other than the thread it was created on."

Reply to this comment
Reply by Steve     September 21 2009 2:49 AM PDT

I don't agree with the advice given by Vijayaragavan. Just ignoring illegal cross-thread calls is a recipe for problems. The correct way is to check whether the call is a cross-thread call and then if necessary marshall the call back onto the main thread using the "Invoke" method. I find the easiest way is first to define a function that adds an item to the listbox:

Private Sub ListAdd(ByVal S as String)
ListBox1.Items.Add(S)
End Sub

Then create a delegate for it. Delegates are just a technique to allow a function to be passed as a parameter to another function:

Private Delegate Sub ListAddDelegate(ByVal S As String)

Now from within your thread you can do this:

If ListBox1.InvokeRequired Then
ListBox1.Invoke(New ListAddDelegate(AddressOf ListAdd), New Object() {"Item to add"})
Else
ListAdd("Item to add")
End If

The "New Object(){}" part creates an array of objects to match the parameters of the Delegate. In this case there is only one parameter, but we still need to make it into an array. "InvokeRequired" tests to see if this is a cross-thread call. If it's not we just call the function directly.

Reply by Vijayaragavan     September 10 2009 3:43 AM PDT

Hi
Use this code start of that function where you are trying to
access the listbox

CheckForIllegalCrossThreadCalls = False

or create a another thread and there update the listbox control




By manson     August 11 2009 12:46 PM PDT

hi, i think this post is great!, but i have a questions about.
I need this source and i'm thinking to put it into web page (aspx), what happen if users close my web page... , can i abort my thread? wich event i must use?

Reply to this comment
By Art     May 25 2009 12:15 AM PDT

hi, fist... this is the best tutorial, but a have a one question:

Sub SquareEventHandler(ByVal Square As Double) _
Handles oSquare.ThreadComplete
MsgBox("The square is " & Square) <-- This works
textbox1.Text = Square <-- this does not work
End Sub

what is the problem ?

sorry for my poor english :(

Reply to this comment
Reply by erwinyn     January 9 2010 5:40 PM PDT

'For art:

Imports System.Threading

'..........................................................................
Public Class Form1

Dim WithEvents oSquare As SquareClass

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
oSquare = New SquareClass
Dim t As New Thread(AddressOf oSquare.CalcSquare)
oSquare.Value = 30
t.Start()
End Sub

Private Sub oSquare_ThreadComplete(ByVal Square As Double) Handles oSquare.ThreadComplete
MsgBox("The square is " & Square)
End Sub

End Class


'..........................................................................
Public Class SquareClass

Public Value As Double
Public Square As Double
Public Event ThreadComplete(ByVal Square As Double)

Public Sub CalcSquare()
SyncLock GetType(SquareClass)
Square = Value * Value
End SyncLock
RaiseEvent ThreadComplete(Square)
End Sub

End Class

Reply by Steve     September 21 2009 3:08 AM PDT

Windows forms are generally not thread-safe. The difference in your example is that the message box has been created by your thread so it is no problem for your thread to use it, but the textbox is on a form created by your main thread and cannot be accessed legally from another thread.

For a solution see my other post to Jasbir Singh.

Because all the controls on a form are (normally) created on the main thread, the invoked function (ListAdd in my example) can update as many other controls on the same form as you wish, rather than needing to make a separate invoke call for each.


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