Multithreading Help

  • Thread starter Thread starter Guest
  • Start date Start date
G

Guest

I trying to learn how multithread works. I found an example in C# and I am
trying to make it work in VB, but I am having problems with it. Could
someone help me to make it work. Here is the code I am working with. Why is
the main thread executed first and them the other?

Imports System.Threading
Public Class Form1

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

thr = New Thread(AddressOf ProcessMsg)
thr.Start()
Call MainThread()
End Sub

Private Delegate Sub WriteMsg(ByVal value As String)

Private Sub PostMsg(ByVal value As String)
Me.TextBox1.Text += "Secondary Thread: " & value & vbNewLine
End Sub

Private Sub ProcessMsg()
Dim i As Long

For i = 0 To 5
Me.Invoke(New WriteMsg(AddressOf PostMsg), New Object()
{i.ToString})
Next
End Sub

Private Sub MainThread()
Dim i As Long

For i = 0 To 5
Me.TextBox1.Text += "Main Thread: " & i.ToString & vbNewLine
Me.TextBox1.Refresh()
Thread.Sleep(350)
Next
End Sub
End Class
 
HLong said:
I trying to learn how multithread works. I found an example in C# and I am
trying to make it work in VB, but I am having problems with it. Could
someone help me to make it work. Here is the code I am working with. Why is
the main thread executed first and them the other?
Click to expand...

Your worker thread is using Control.Invoke to call back to the main
thread. That's exactly the right thing to do, but in this case you've
kept the main thread busy doing other things first. The calls to Invoke
will only be made once the UI thread has finished processing
Button1_Click.

If you manually made calls to Application.DoEvents I suspect you'd see
the secondary thread in action earlier - but DoEvents is a generally
bad idea.
 
Thank you.
I am just starting to learn about multithreading. Does this mean that the
main thread has priority over any other thread? I have tried setting the
priority of the second thread to highest and still did not make any diff. If
I start two threads from the control, then both are executed, how I would
expect. What would be the best approach to execute code simultaneously from
a form? TIA
 
HLong said:
I am just starting to learn about multithreading. Does this mean that the
main thread has priority over any other thread?
Click to expand...

No - although GUI threads are a high priority. The problem is using
Control.Invoke. Your secondary thread is getting as far as the call to
Invoke, but then it's got to wait for the UI to finish before it can
get any further.
I have tried setting the
priority of the second thread to highest and still did not make any diff. If
I start two threads from the control, then both are executed, how I would
expect. What would be the best approach to execute code simultaneously from
a form? TIA
Click to expand...

Don't put *either* of the tasks in the main GUI thread - the UI thread
should remain responsive at all times, if possible. Start two different
threads to do the work, and you'll see both executing.
 
Jon Skeet [C# MVP] pisze:
No - although GUI threads are a high priority. The problem is using
Control.Invoke. Your secondary thread is getting as far as the call to
Invoke, but then it's got to wait for the UI to finish before it can
get any further.
Click to expand...
What you are trying to say is that Invoke method adds a message to
message queue and waits till it's executed, and the execution of the
queue is in the GUI thread which is busy, right?
So doing DoEvents from time to time in the GUI thread would solve the
problem.
 
Thank you very much.

Jon Skeet said:
No - although GUI threads are a high priority. The problem is using
Control.Invoke. Your secondary thread is getting as far as the call to
Invoke, but then it's got to wait for the UI to finish before it can
get any further.


Don't put *either* of the tasks in the main GUI thread - the UI thread
should remain responsive at all times, if possible. Start two different
threads to do the work, and you'll see both executing.
Click to expand...
 
Doker said:
Jon Skeet [C# MVP] pisze:
No - although GUI threads are a high priority. The problem is using
Control.Invoke. Your secondary thread is getting as far as the call to
Invoke, but then it's got to wait for the UI to finish before it can
get any further.
Click to expand...
Click to expand...
What you are trying to say is that Invoke method adds a message to
message queue and waits till it's executed, and the execution of the
queue is in the GUI thread which is busy, right?
Click to expand...

Yes, that's right.
So doing DoEvents from time to time in the GUI thread would solve the
problem.
Click to expand...

Yes, but in a nasty way which could lead to re-entrancy issues.
DoEvents should be avoided where possible.
 
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