I use multithreading in my program and I want to create a button to check if any of the threads is still running. If there are no threads running in the background, I want the button to do a specific job like run another thread.
I've written the following code on the button:
If thread1 Or thread2 Or thread3 is Nothing Then
thread2 = New Thread(AddressOf Me.thread2_engine)
thread2.Start()
Else If thread2.IsAlive = True Then
MsgBox("Processing right now, Please wait")
End If
Why can't I use an Or operation inside the if statement?
If use Or like that, you are doing a bitwise Or on the variables. I think what you mean is
If (thread1 Is Nothing) Or (thread2 Is Nothing) Or (thread3 Is Nothing) Then
You might want to use OrElse instead of Or as that will avoid checking the later conditions once one of them is true.
Related
I have a bug in my application which is the same as here which this person was running into the same problem. My application is multi threaded where the worker thread is updating the Waveformgraph on the UI. I believe that is where my problem is and why, periodically, and on occassion I get a big red X in at least one of my waveformgraph objects when running the application. From reading and research, I need to use an Invoke or BeginInvoke method? Can someone please explain better and provide a sample code that is relevant to my code? The samples that I've found so far still have me hazy on how I need to do this or what I need to do. Thank you for your help.
This code is on the swScopeOnOff click event, main thread.
thread2 = New System.Threading.Thread(AddressOf dataAcquiring)
thread2.Start()
This code is in dataAcquiring Sub
Public Sub dataAcquiring()
'While Scope switch is on, stream each Ai channel's data continuously to its respective WaveForm graph
Do While swScopeOnOff.Value = True
data = reader.ReadWaveform(readRate)
i = 0
For Each WaveformGraph In WFGS
WaveformGraph.PlotWaveformAppend(data(i)) 'This line is updating the UI's waveform graphs
i += 1
Next
i = 0
Loop
End Sub
Proper, thread-safe invocation is actually not as hard as one might think (not even for thread-safe events, but that's irrelevant for this question).
I would recommend you to use the normal Invoke method, such as Me.Invoke() (where Me is the current form, if not, use Form1 or whatever it's called instead). Using BeginInvoke() may be asynchronous but it stacks memory usage and can cause memory leaks if EndInvoke() is not called correctly.
If you target .NET 4.0 or higher you can simply do like this:
Me.Invoke(Sub() WaveformGraph.PlotWaveformAppend(data(i)))
However if you target .NET 3.5 or lower it requires a few more lines of code.
'Outside your Sub.
Delegate Sub WaveformAppendDelegate(ByRef WaveformGraph, ByRef data)
'Create a new sub.
Public Sub AppendData(ByRef WaveformGraph, ByRef data)
WaveformGraph.PlotWaveformAppend(data)
End Sub
'Inside your sub, when you're going to invoke.
Me.Invoke(New WaveformAppendDelegate(AddressOf AppendData), WaveformGraph, data(i))
My project is vb.net 2010 windows desktop form.
So far, single threaded (default).
If a SUBroutine has a for...next loop in it that is running, what happens if a buttonclick event is fired and within that event a variable is changed? Like: does program execution leave the loop that was running? Or does it continue to run while that variable is changed by the buttonclick event?
What I'm aiming for:
If someone clicks the button, blnRequestStop is set to True.
Within that for...next loop, just before the "next" it checks blnRequestStop. If true then it will exit the "for" loop.
I'm guessing I need to use threads? Can anyone give me a simple example, please?
EDIT:
This code below seems to be working fine. But maybe you all see a problem?
If (btnProcess.Text = "Done!") Then
End
ElseIf (btnProcess.Text = "IMPORT") Then
bRequestStop = False
t1 = New Thread(AddressOf ProcessDo)
t1.Start()
Else
t2 = New Thread(AddressOf MyInterrupt)
t2.Start()
End If
Here is the short version of what ProcessDo and MyInterrupt do:
Private Sub ProcessDo()
For each X in blahblah
'do stuff (yes, includes interface)
if (blnInterrupt) then exit For
Next X
End
End Sub
Private Sub MyInterrupt()
blnInterrupt=true
End Sub
Yes, you probably want to do the long-running task on a background thread. Here's a code sample including how you'd get the results back to the UI thread when you're done (otherwise you'll get errors about Cross-thread operation not valid).
ThreadPool is a nice way to do some work on a background thread. You could set stopIt = True in the button click event for the stop button.
ThreadPool.QueueUserWorkItem(
Sub()
For Each thing In things
If stopIt Then Exit Sub
'Do the stuff!
Next
'We're done, update UI
Me.UpdateUI("All done!")
End Sub)
To safely update the UI, you'll need to make sure you get back to the UI thread.
Public Sub UpdateUI(result As String)
If Me.InvokeRequired() Then
'If we aren't on the UI thread, invoke this function on the UI thread
Me.BeginInvoke(Sub() UpdateUI(result))
Exit Sub
End If
'Update UI here
lblResult.Text = result
End Sub
Execution is 'stopped' (in a way) until the Loop finishes him job, so yes, you need to multi-thread.
It it is not a very long operation where you need to update UI controls during the Loop then you just can use Application.DoEvents inside the loop to be able to use other controls as normally in the application when FOR loop is working, but I advise you that this will have a negative impact on UI performance, but if it's not a long duration loop then you maybe would consider to use DoEvents instead introduce into multi-threading it's just an alternative, not recommended but, you can use it.
PS: Forgive my English
I'm trying to figure why my form freezes up when executing some code. I also can't minimize or move the form. Is it because of the WaitForExit being used in the process?
The below code is tied to a button click.
If Checkbox1.checked = True Then
Call Test()
End If
If Checkbox2.checked = True Then
Goto NextStep
Else
Goto StopProcessing
End If
Here is the test sub I'm calling. Calls an exe with an optional argument.
Using psinfo As New Process
psinfo.StartInfo.FileName = "C:\Temp\Test.exe "
psinfo.StartInfo.Arguments = Arg1
psinfo.StartInfo.WindowStyle = ProcessWindowStyle.Hidden
psinfo.Start()
psinfo.WaitForExit()
End Using
The WaitForExit was added (so I thought) to not process the next statement (next statement being the If statement for Checkbox2) until the process was complete. Is this not the case?
The WaitForExit was added (so I thought) to not process the next statement (next statement being the If statement for Checkbox2) until the process was complete.
When you call WaitForExit, it will block until the process (Test.exe) completes.
Since you're running this on the user interface thread, it will cause your form to "freeze" until the process completes fully.
If you need this to not occur, you would need to wait on a background thread. You could, potentially, move this code into a BackgroundWorker and use it to synchronize with your main window - but you will need to handle "waiting" for the process to finish in a different manner (ie: disable your UI up front, run the process, re-enable when complete).
Note that, with the Process class, another alternative would be to add EnableRaisingEvents on the process, then adding a handler to Process.Exited. This will let you not WaitForExit(), but instead get notified via an event when the process completes.
This Question has 2 parts. I am new to multithreading and so I want to firstly check if my logic is correct and then I want to find out how to handel erros in multithreading.
Q1: I have an application that calls SQL database obtaining information from 2 datatables, this info is then combined in a final display. Without multithreading, I call each SQL select to populate a dataset one after the other. With multithreading I call the more complex SQL first as a separate thread and then the less complex SQL call in the main thread second. I am trying to cut down the load time of both by doing them concurently.
(I realise that strictly I should do both as backround tasks to free up the UI, for me its small steps first)
Anyway the code looks little like this
Dim ThreadLoad_Longer_Data As Thread
ThreadLoad_Longer_Data = New Thread(AddressOf Me.Fill_LongerSQL)
ThreadLoad_Longer_Data.IsBackground = True
TThreadLoad_Longer_Data.Start()
'Execute some code here for the second SQL call in main thread
'Then stop the main prosess to wait for the finish of the of the background
ThreadLoad_Longer_Data.join
Im assuming that the .Join statment will infact stop the main thread and will wait for the other one to finish ? Is this correct ?
If so it brings me to the second part.
Q2. What happens if the first thread dosent finish? Like through an error ? How do I handle this situation ?
Thank you
Yes, calling ThreadLoad_Longer_Data.Join will stop the execution of the calling thread (the one that executes the code calling the Join) till the ThreadLoad_Longer_Data ends its execution.
If, inside ThreadLoad_Longer_Data, you have an unhandled exeception, the result is the ending of the thread and thus the resume of the execution of the calling thread.
Sub Main
Try
Console.WriteLine("Start of the main thread")
Dim ThreadLoad_Longer_Data As Thread
ThreadLoad_Longer_Data = New Thread(AddressOf Me.Fill_LongerSQL)
ThreadLoad_Longer_Data.IsBackground = True
ThreadLoad_Longer_Data.Start()
ThreadLoad_Longer_Data.Join
Console.WriteLine("End of the main thread")
Catch x as Exception
Console.WriteLine(x.Message)
End Try
End Sub
Sub Fill_LongerSQL()
Console.WriteLine("Before the exception")
dim y as integer
for x = 0 to 1000000000
y = y + 1
next
Throw new Exception("This is an unhandled exception")
' this will never executed
Console.WriteLine("After the exception")
End Sub
Okay so here's the thing:
I have a linq query which loads approx. 1000 lines into a variable, during that process I want to display a progressbar, not necessarily stating the percentage, can be marquee style, doesnt matter.
This progressbar is on a modal form to precent the user from interacting with the app for the time the query's running.
Now here's my code:
Private Sub LoadBar()
Try
Dim load As New frmLoadbar
load.Text = "Loading bunch of data..."
load.ShowDialog()
Catch e As Threading.ThreadAbortException
Threading.Thread.ResetAbort()
End Try
End Sub
In another sub:
Dim myThreadDelegate As New Threading.ThreadStart(AddressOf LoadBar)
Dim th As New Threading.Thread(myThreadDelegate)
th.Name = "TimeConsuming"
th.Start()
Dim XY = db.Table.GetEnumerator
While XY.MoveNext
Dim item As New ListViewItem
item.Text = XY.Current.Name
item.Tag = XY.Current
ListBox1.Items.Add(item)
End While
Autos.Dispose()
Try
th.Abort()
Catch ex As Exception //here's where i 'swallow the re-thrown exception
End Try
Not thats one of the ugliest code i've ever written.It works i just dont want that rethrown exception.
Some explanation:
I want the modal form to close after the query is done.
For that reason I 'abort' the thread running the form.
Since aborting a thread throws a double-exception i have to 'swallow'
that exception.
Now i know i could implement this like the following:
Coding a loop into the form holding the progressbar, which checks
periodically for a boolean's value, and if its true the form could
close itself.
From the other form - on the worker thread - i could change that
booleans value to true after the query's finished.
But here comes my question:
Whats the best way to implement this?
I know it can be done with a background worker, which has been
specifically invented for this reason, but can i use the background
worker as the thread to show the progressbar?
If not (and i have to run the query on the background worker and
showing the modal form from my original form), would that mean that
the query would "work in the background"?
Would that mean that the query would be slower?
I've looked into other tutorials, but for one reason or another, either i wasnt able to copy it (due to complexity) or I wasn't convinced that it was better than this.
Thank you for your time you took to answer.
You could show the modal form and then run a BackgroundWorker from that form. The progress and completed events would be on the UI thread so you can update a progress bar while it is running and close the form in the completed event handler.
Okay, so for future reference, if someone needs clear help with code samples, Microsoft has it (thats a first..)
You can download it here:
Multithreading
Assuming Windows Forms, you do this with a BackgroundWorker component.