multi threading - add more threads and continue the operation - vb.net

Question

ok here is my code :
For i = 0 To 10
Dim tTemp As Threading.Thread = New Threading.Thread(AddressOf dwnld)
tTemp.IsBackground = True
'tTemp.Start(geturl)
lThreads.Add(tTemp)
'MsgBox(lThreads.Item(i).ThreadState)
Next
I create a list of threads with 10 threads, assign them a function, properties and add them to the list.
'While ListBox2.Items.Count > 0
For i = 0 To lThreads.Count - 1
If (lThreads.Item(i).ThreadState = 12) Then
If (ListBox2.Items.Count > 0) Then
lThreads.Item(i).Start(geturl)
If (i = lThreads.Count - 1) Then
i = 0
End If
Else
Exit For
End If
'MsgBox(lThreads.Item(i).ThreadState)
ElseIf (lThreads.Item(i).ThreadState = 16) Then
lThreads.RemoveAt(i)
Dim tTemp As Threading.Thread = New Threading.Thread(AddressOf dwnld)
tTemp.IsBackground = True
lThreads.Add(tTemp)
If (i = lThreads.Count - 1) Then
i = 0
End If
End If
Next
What's happening is, i see the threads stop after the function dwnld is completed. So i first check for the state (12 means background and unstarted). On case 12 start the thread and in case 16 (stopped) remove that particular thread and add a different thread like i add 10 above.
Also there is a check when the i counter reaches last number, restart the whole loop by assigning i=0.
The program downloads some web pages, the url is passed from the listbox2. The geturl will pass the url and remove it from the list. So when the listbox is empty, exit the for loop.
But the above code is running for only 11 times and it does not restart. I tried using a lable and goto but it simple hangs.
Can anyone tell me what to do?
What i want is to maintain 10 threads to keep downloading the web pages and when the list is empty, exit the function.

Answer 1

Trying to manually manage your own custom pool of threads is probably the wrong approach here. Use ThreadPool.QueueUserWorkItem or preferrably the new Task class. The thread pooling is managed for you which greatly simplifies the code. Completely scrap this code and start over using one of the techniques I just mentioned. If you run into problems implementing either of these techniques then post a more specific question.

Answer 2

Micro-management of threads is, well, just a really bad idea. The moment I see anyone trying to maintain a list of threads that are continually created, terminated and destroyed I just know they are doomed. I have seen experienced professionals trying to do it - it's fun looking on, waiting for the inevitable spectacular failure after months of trying to fix the unfixable.
Thread pools are, typically, nothing of the sort. They are usually a pool of tasks - task class instances on a producer-consumer queue - that several threads feed off as and when they are free to do work. The work threads auto-manage themselves by getting new tasks themselves when they have finished with the old one - no need for any higher-level micro management.
Listen to #Brian - forget managing lists of threads, checking their state and all that gunge. It'll just make you ill. Go with ThreadPool.QUWI or Tasks.