Multithreading in VB6

Posted on 2002-04-18
Last Modified: 2012-06-21
Does anyone know how to do multithreading in VB6?  For example, if I want to call a public function Process1()during form load, I want the form keep loading without waiting on Process1() to finish.  Actually, I have a case the Process1() will be running during whole run time.  It is like sth below:

Public Function Process1(arg1 as String)
do while True
End Function

So, is there a way to use multithreading for this?  I'd like to use it inside of the code, instead of creating an ActiveX.

Working sample code needed please.


Question by:hxia
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by:Éric Moreau
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It will be hard for a VB6 app to get multi threading. BTW, supports it natively.

have a look at

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spyworks from may help you to do it.

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Accepted Solution

ameba earned 150 total points
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1. In Form_Load start your "long task" asynchronously.
2. In the loop in Process1() add enough DoEvents lines (e.g. 10 DoEvents per second).

For 1 you'll need a timer.  To prevent reentering Process1() or to signal task has finished, you'll need a flag (Boolean variable).

' in Form_Load
    ReadSomething ...
    Timer1.Enabled = True    ' this will start Process1
    DoSomething ...

' in Timer1_Timer
    Timer1.Enabled = False
    If bExecuting Then Exit Sub
    bExecuting = True   ' set flag, defined in a module or form
    Call Process1()
    bExecuting = False  ' reset flag

' in Process1
Public Function Process1(arg1 as String)
    Dim somecounter As Long
        somecounter = somecounter + 1
        If somecounter Mod 100 = 0 Then
            DoEvents    ' allow other tasks
        End If


        If bDone Then Exit Do
End Function
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DoEvents is the easiest solution to my problem, but not the most efficient.  

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Moondancer - EE Moderator

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Visual Basic allows you to launch components in background threads when using ActiveX EXE servers, however it does not allow this when using ActiveX DLL's or regular exe. Some programmers have attempted to use the CreateThread API to perform background thread operations with VB, but the results range from unsatisfactory to disastrous (refer to our article "A Thread to Visual Basic" for details).

The dwBackThread component makes it easy for the first time to launch objects in background threads in your ActiveX DLL components which support background (asynchronous) operations.

The dwBackThread component follows all of the COM threading rules necessary to make an object work reliably with Visual Basic. It performs cleanup tasks to allow your background thread to terminate correctly even if you don't release all of your objects properly.

The Desaware background thread component is included with SpyWorks Professional 6.2 and later and the NT Service Toolkit 1.0 and later.

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Based on review and feedback from other moderators, the grade is changed.

Please see below.

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ameba's answer is insightful but critical section is not considered so when accessing to shared variable, be careful. To avoid this, you have launch thread i.e. timer that do not use shared variable.

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