Why doesn't this multithreaded VB.NET (2010 Express Edition) program work properly? - vb.net

Question

I'm trying to make a program of mine into a multithreaded application, but I've hit a pair of snags that I documented in the following code. Any help that I can get with this to make it behave properly would be greatly appreciated so I can expand this stub into a more efficient version of my existing application.
Thank you for any advice you have on this matter.
- Aaron
Imports System.Threading
Public Class frmMain
''' <summary>Initializes the multithreaded form</summary>
Private Sub Initialize() Handles MyBase.Load
AddThread(AddressOf Update_UI)
running = True
For Each Thread In ThreadPool
Thread.IsBackground = True
Thread.Start()
Next
End Sub
''' <summary>Terminates the multithreaded form</summary>
Protected Overrides Sub Finalize() Handles MyBase.FormClosing
running = False
For Each Thread In ThreadPool
Thread.Join()
Thread = Nothing
Next
End Sub
''' <summary>Adds a worker thread to the ThreadPool</summary>
''' <param name="pointer">The AddressOf the function to run on a new thread.</param>
Private Sub AddThread(ByRef pointer As System.Threading.ParameterizedThreadStart)
Dim newthread As Integer
If ThreadPool Is Nothing Then newthread = 0 Else newthread = ThreadPool.GetUpperBound(0) + 1
ReDim Preserve ThreadPool(newthread)
ThreadPool(newthread) = New Thread(pointer)
End Sub
''' <summary>Updates the User Interface</summary>
Private Sub Update_UI()
'HELP: The commented out lines in this subroutine make the program work incorrectly when uncommented.
'HELP: It should echo 'output' to the titlebar of frmMain, but it also makes the form unresponsive.
'HELP: When I force the form to quit, the 'termination alert' does not trigger, instead the application hangs completely on Thread.Join (see above).
'HELP: If I remove DoEvents(), the form is unable to be closed...it simply goes unresponsive. Shouldn't the multithreading keep us from needing DoEvents()?
'If Me.InvokeRequired Then
' Me.Invoke(New MethodInvoker(AddressOf Update_UI))
'Else
While running
Dim output As String = System.DateTime.Now + " :: Update_UI() is active!"
Debug.Print(output)
'Application.DoEvents()
'Me.Text = output
End While
Debug.Print(System.DateTime.Now + " :: Termination signal recieved...")
'End If
End Sub
Delegate Sub dlgUpdate_UI()
Private ThreadPool() As Thread
Private running As Boolean
End Class

Answer 1

Yes, none of what you tried can work properly. You correctly identified the need to use Control.Invoke() to run the Me.Text assignment on the main thread. This is what is going wrong:
Your Invoke() call makes the entire method run on the main thread. It will start executing the loop and never exit. Your form goes catatonic since it can't do anything else anymore, like repaint the caption bar to show the changed text or respond to user input
The DoEvents call makes the form come back alive but now you've got a new problem: the user can close the window and your code keeps running. The running flag will never be set to false so the program won't stop. The user interface is gone though. Code would normally bomb on an ObjectDisposedException but not in your specific case, the Text property is stored in a private variable
You could alter the code so that only the Me.Text assignment runs on the main thread. But now you've got a new problem: the main thread will get pummeled by invoke request and doesn't get around to doing its regular (low priority) duties anymore. It goes catatonic. The essential problem is that you are trying to update the caption bar way too fast. There's no point, the user cannot read that fast. Update 20 times per second is plenty and looks smooth to the human eye
Do not use the Finalize() method for tasks like this, the code can easily trigger the 2 second finalizer thread time-out, bombing your program.
Do consider using the BackgroundWorker class, it takes care of some of these details.

Answer 2

It's the do while loop that is burning up all your cycles so you are loosing the battle and keeping the procesor busy no matter how many threads you use. Something like the following will be better suited for what you are trying to achieve.
Imports System.Threading
Public Class Form1
Private t As New Timer(AddressOf DoTimer, Nothing, 1000, 1000)
Private Sub DoTimer(ByVal state As Object)
UpdateUi()
End Sub
''' <summary>Updates the User Interface</summary>
Private Sub UpdateUi()
If InvokeRequired Then
Invoke(New DlgUpdateUi(AddressOf UpdateUi))
Else
Dim output As String = DateTime.Now & " :: Update_UI() is active!"
Debug.Print(output)
Text = output
End If
End Sub
Delegate Sub DlgUpdateUi()
Private Sub Form1_FormClosing(sender As Object, e As System.Windows.Forms.FormClosingEventArgs) Handles Me.FormClosing
t.Dispose()
End Sub
End Class

Answer 3

If I have said once I have said it a million times. Using Invoke, while useful in many situations, is abused and way overused. If all you want to do is have the progress of the worker threads displayed to a user then using Invoke is not always the best option. And it does not look like the best option here either.
Instead, publish the status text you are assigning to output into a variable that can be accessed via the UI thread. Then use a System.Windows.Forms.Timer to periodically poll its value at a more reasonable rate...maybe every 1 second or so. The Tick event already runs on the UI thread so you can immediately begin using this value to display to the end user by manipulating the various UI controls.
Strings are really easy to pass around from thread to thread because they are immutable which means they are inherently thread-safe. The only thing you really have to worry about is making sure the UI thread sees the most recent reference published to the shared variable. In C# you would use the volatile keyword for this. In VB you can use Thread.VolatileRead from the UI thread and Thread.VolatileWrite from the worker thread. Of course, if you are more comfortable wrapping the reads and writes in a SyncLock that is perfectly acceptable as well.