Question

I’m trying to run a multi-threaded console app in VB and am having thread cross-over. Basically I want to run 5 threads, have them continually access a queue, process, and repeat until there’s nothing left. When all threads have processed I want them to do something else. I’m attempting to use SyncLock to prevent multiple threads from accessing but it does not seem to be working. Any help would be appreciated!

Dim iThread As Integer
Dim manualEvents(4) As ManualResetEvent

Sub Main()
    For i = 0 To 4
        manualEvents(i) = New ManualResetEvent(False)
        ThreadPool.QueueUserWorkItem(AddressOf DoOne)
    Next

    For Each handle As WaitHandle In manualEvents
        handle.WaitOne()
    Next

    ' do other stuff
EndSub

Private Sub DoOne()
    Dim lockObject As New Object()
    SyncLock (lockObject)
        If QueryQueue.DoOne() Then
            DoOne()
        Else
            manualEvents(iThread).Set()
            iThread = iThread + 1
        End If
    End SyncLock
End Sub

Answer 1

The problem is with the locked resource, you’re using lockObject as a synchronization lock resource which should be shared accros threads.
You have to make it an instance field.

Private Shared lockObject As New Object()
Private Sub DoOne()
  SyncLock (lockObject)
    If QueryQueue.DoOne() Then
        DoOne()
    Else
        manualEvents(iThread).Set()
        iThread = iThread + 1
    End If
  End SyncLock
End Sub