[RESOLVED] Wait for process to finish BUT keep VB app running as normal?

[christopherpm]

I am creating a batch processing interface for a program that doesn't currently support batch processing.

The user clicks on a form button to trigger the batch processing. When they click on the button, obvioulsy this calls the subroutine mybutton_click()

Within the subroutine, my code to start the external app is:-

Dim myProcess As Process = System.Diagnostics.Process.Start(myProgram, myProgramParams)

because 'myProcess' contains the name of the running process, I can create a loop within the subroutine to check if the process is running (using Process.GetProcessByName), and if it is, wait for it to complete before starting the next batch entry.

However, this method means that my program has to continuosly loop within the subroutine until the external process is no longer running and no more batch entries exist before End Sub is reached.

What I really need to do is this:-

1) Process the first entry in the batch but wait until the process completes before starting the next batch entry - I don't want any more than one instance of the external program running at any one time.

2) While any batch entry is running, I want control to return to my program so that the user can do other things - but obviously, when the external process is complete, I need my program to start the next batch entry if there is one...

How could I go about this? (I hope the above makes sense)

Many thanks...

[si_the_geek]

Thread moved from CodeBank forum (which is for you to post your code examples, not questions)

[Atheist]

Welcome to the forums.
The first thing you need to do is to set the EnableRaisingEvents property to True.

myProcess.EnableRaisingEvents = True

Then add a handler for the Exited event:

AddHandler myProcess.Exited, AddressOf ProcessExited

And of course, you need an eventhandler subroutine:

Private Sub ProcessExited(ByVal sender As Object, ByVal e As System.EventArgs)
 
End Sub

Just add your logic in the ProcessExited method. Voila

[MaximilianMayrhofer]

You may also want to consider using a separate thread.

[christopherpm]

You may also want to consider using a separate thread.

This is the method that I am attempting without success. This is what I have so far.

Within the myButton_Click() routine, I have the following lines:-

Dim thrMyThread As New System.Threading.Thread(AddressOf StartBatch)
thrMyThread.Start()

This obviously calls the StartBatch() routine. This includes the following line, which throws up an exception ONLY when being called using the Thread start method.

strParams = " /file=" & ListView1.Items(0).SubItems(1).Text

The exception message reads:-

System.InvalidOperationException was unhandled
  Message="Cross-thread operation not valid: Control 'ListView1' accessed from a thread other than the thread it was created on."
  Source="System.Windows.Forms"
  StackTrace:
       at System.Windows.Forms.Control.get_Handle()
       at System.Windows.Forms.ListView.ListViewNativeItemCollection.DisplayIndexToID(Int32 displayIndex)
       at System.Windows.Forms.ListView.ListViewNativeItemCollection.get_Item(Int32 displayIndex)
       at System.Windows.Forms.ListView.ListViewItemCollection.get_Item(Int32 index)
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.runTryCode(Object userData)
       at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
       at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException:

HOWEVER if I call the StartBatch() routine without trying to use threading, then that line works as I would expect... This is my first time with VB 2008 so I'm afraid that I don't know where I'm going wrong... (I've only used VB6 until now)

[MaximilianMayrhofer]

The exception message is telling you exactly what you are doing wrong. You are trying to access the ListView, which is on your UI thread, from your Process thread. I'm not entirely sure how to solve it, but you could try pushing the Item value into a global string variable and then reading it, like so:

Public Class YourForm
    Private LVItem As String

    Private Delegate Sub ItemToStringInvoker(ByVal ItemIndex As Integer, ByVal SubItemIndex As Integer)

    Private Sub ItemToString(ByVal ItemIndex As Integer, ByVal SubItemIndex As Integer)
        If Me.ListView1.InvokeRequired Then
            Me.ListView1.Invoke(New ItemToStringInvoker(AddressOf GetListViewItem), ItemIndex, SubItemIndex)
        Else
            LVItem = ListView1.Items(ItemIndex).SubItems(SubItemIndex).Text
        End If
    End Sub
End Class

Then change this line:

strParams = " /file=" & ListView1.Items(0).SubItems(1).Text

to:

ItemToString(0, 1)
strParams = " /file=" & LVItem

Of course, you could always put a BackgroundWorker in your components at design time, then do something like this:
Change your StartBatch() Routine to include a Parameter, strItem:

Private Sub StartBatch(ByVal strItem As String)
    strparams = " /file=" & strItem
End Sub

In your button click event:

BackgroundWorker1.RunWorkerASync(ListView1.Items(0).SubItems(1).Text)

In your BackgroundWorkers Do_Work event:

Start_Batch(DirectCast(e.Argument, String))

very clean.

[christopherpm]

Thanks - I'll give that a go shortly. But can I ask - with regards to the second part of my problem listed in my initial post above, what would be the best way to get my app to keep checking to see if the external program is running so that I can invoke the next entry in the batch only once the current batch entry has completed? One thing I want to be careful of is slowing the program down too much...

[Atheist]

Theres no need for a separate thread if you're handling the process' Exited event.

[MaximilianMayrhofer]

How is the exited event raised, if i may ask? Is the process started in another thread automatically when you do Process.Start()? If so, where is the application actually checking to see if the process has exited?

[jmcilhinney]

How is the exited event raised, if i may ask? Is the process started in another thread automatically when you do Process.Start()? If so, where is the application actually checking to see if the process has exited?

The process isn't started in another thread. It's a whole separate process. Threads run within processes, not the other way around.

The Process class is a managed wrapper for a Windows process. When the process exits the Process object knows because it gets notified by the OS via the process handle. It then passes that notification on to you as a managed event.

[MaximilianMayrhofer]

Thanks jmc, that helped alot. I think I know exactly where i'll be able to apply this knowledge.

[christopherpm]

Thanks MaximilianMayrhofer - my code is now working just as I want it to...