VB.NET : Thread is running or terminated; it cannot restart - vb.net

Question

I've encounter this error when using multi-threading. I'm new with it, in my windows application this code works. But I transfer it to windows services I've received "Thread is running or terminated; it cannot restart." I'm using System.Timers.Timer instead of System.Windows.Forms.Timer as recommended when creating it in Windows services. This windows services will export some XML file from database, so I need a timer. So time to time, it will check if there's a new products or customer in the database which reads the function below. By default, I've hard coded the time to 1min for testing. Also, I've created a boolean variable if the function is not finish yet. It will not override.
Here's my code :
Dim oIsproc_BP As Boolean
Dim oIsproc_ItemMaster1 As Boolean
Dim thrd As Thread
Protected Overrides Sub onstart(ByVal args() As String)
tmr.Interval = 1000
AddHandler tmr.Elapsed, AddressOf tmr_Elapsed
tmr.Start()
End Sub
Private Sub tmr_Elapsed(sender As Object, e As Timers.ElapsedEventArgs) Handles tmr.Elapsed
oIsproc_BP = False
oIsproc_ItemMaster1 = False
tSecItemMaster.Interval = 60000'oInterval(0)
AddHandler tSecItemMaster.Elapsed, AddressOf tSecItemMaster_Elapsed
tSecItemMaster.Start()
tSecCustomer.Interval = 60000'oInterval(2)
AddHandler tSecCustomer.Elapsed, AddressOf tSecCustomer_Elapsed
tSecCustomer.Start()
tmr.Stop()
End Sub
Private Sub tSecItemMaster_Elapsed(sender As Object, e As Timers.ElapsedEventArgs) Handles tSecItemMaster.Elapsed
If Not oIsproc_ItemMaster1 Then
oIsproc_ItemMaster1 = True
thrd = New Thread(DirectCast(Function() oItemMaster(), ThreadStart))
thrd.Start()
End If
Return
End Sub
Private Sub tSecCustomer_Elapsed(sender As Object, e As Timers.ElapsedEventArgs) Handles tSecCustomer.Elapsed
If Not oIsproc_BP Then
oIsproc_BP = True
thrd = New Thread(DirectCast(Sub() oBPartners(, "C"), ThreadStart))
thrd.Start()
End If
Return
End Sub
And for my function :
Private Function oItemMaster(Optional ByVal FirstLoad As Boolean = False, Optional oType As Integer = 1)
''My code here
oIsproc_ItemMaster1 = False
End Function
Private Sub oBPartners(Optional ByVal FirstLoad As Boolean = False, Optional CardType As String = "C")
''My code here
oIsproc_BP = False
End Function

Answer 1

You have a race - you're using a single variable (thrd) to hold one of two instances of a Thread that you might create. Consider both timers firing at the same time, and the threads that service the timers being interleaved as follows:
Timer 1 (tSecItemMaster_Elapsed) Timer 2 (tSecCustomer_Elapsed)
If Not oIsproc_ItemMaster1 Then
oIsproc_ItemMaster1 = True
If Not oIsproc_BP Then
oIsproc_BP = True
thrd = New Thread(...)
thrd = New Thread(...)
thrd.Start()
End If
Return
thrd.Start()
End If
Return
And that's why you get the error message - both timers are trying to start the single Thread object that was created inside tSecItemMaster_Elapsed, and the Thread created inside of tSecCustomer_Elapsed is never started. Other inter-leavings will introduce similar issues.
A quick fix would be to create a separate field for storing each thread. I think you may still have a couple of race conditions but they're not leaping out at me at the moment.