vb.net problems with invoke - vb.net

Question

I have a thread that runs background jobs and is required to update the GUI once in a while. My program has been designed so that when the user clicks off of a form, the thread and background operations still run, yet the controls have been disposed (for memory management purposes).
I have been using Invoke() and "If Control.Created = True" to make sure that the thread can successfully update the controls without running into any exceptions. However, when the form is recreated, all "Control.Created" values are false and Invoke() fails with "{"Invoke or BeginInvoke cannot be called on a control until the window handle has been created."}"
My guess is that this has something to do with the fact that when the form is recreated it is assigned different handles and that the "Invoke()" is looking at the old handle. SO my question is, how do I fix this?
EDIT: As per requested, the code for opening the form and where the bg thread works from
Opening the DropLogMDIalt form is simply
FormCTRL.Show()
The Background Thread runs when the control is modified so that the NumericUpDown is more than 0 (so that there is something to countdown from)
Private Sub NLauncherTerminateInput_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles DLScanInterval.ValueChanged
If DLScanInterval.Created = True Then
DLTimerControlValue = DLScanInterval.Value
If DLTimerControlValue = 0 Then
CancelDropLogTimer()
Else
If DLScanIntervalControl.Active = False Then
BeginDropLogTimer()
End If
End If
End If
End Sub
Public Sub BeginDropLogTimer()
Dim N As New Threading.Thread(AddressOf DropLogTimerIntervalThreadWorker)
N.Start()
DLScanIntervalControl.ThreadID = N.ManagedThreadId
DLScanIntervalControl.Active = True
End Sub
Public Sub CancelDropLogTimer()
DLScanIntervalControl.Active = False
End Sub
Public Sub DropLogTimerIntervalThreadWorker()
DLScanTimerSecondsLeft = DLTimerControlValue * 60
Dim s As Integer = DLTimerControlValue
Do Until 1 = 2
DLScanTimerSecondsLeft = DLTimerControlValue * 60
Do Until DLScanTimerSecondsLeft <= 0
If Not (DLTimerControlValue = 0 Or DLScanIntervalControl.CancelPending = True) Then
Else
Exit Sub
End If
If Not DLTimerControlValue = s Then
DLScanTimerSecondsLeft = DLTimerControlValue * 60
s = DLTimerControlValue
End If
Dim ToInvoke As New MethodInvoker(Sub()
Timer(DLScanTimerSecondsLeft, ":", DLScanIntervalTB)
End Sub)
If (Me.IsHandleCreated) Then
If (InvokeRequired) Then
Invoke(ToInvoke)
Else
ToInvoke()
End If
End If
Threading.Thread.Sleep(1000)
DLScanTimerSecondsLeft -= 1
Loop
CompareScan = True
PerformScan()
Loop
End Sub
The thread is simply called by declaring a new thread.thread, however, I have created a class and a variable that the thread uses to check if it should still be running or not (similarly to how a backgroundworker would) this is illustrated by the "DLScanIntervalControl.CancelPending"
The form is then closed later by
Form.Close()
It can be reopened if the user clicks on a label that then uses the same method as shown above (FormCTRL.Show())

Answer 1

From MSDN:
"If the control's handle does not yet exist, InvokeRequired searches up the control's parent chain until it finds a control or form that does have a window handle. If no appropriate handle can be found, the InvokeRequired method returns false."
In other words, you need to verify that the handle is created and then check if invoke is required.
If(Me.IsHandleCreated) Then
If(Me.InvokeRequired) Then
'...
Else
'...
End If
End If

Answer 2

I had a similar error when trying to use delegates to update controls on a form in another thread. I found that the handle is only created when it's "needed". I'm not sure what constitutes "needed", but you can force it to create the handle by accessing the Handle property of the object.
What I had done in my application is this:
' Iterate through each control on the form, and if the handle isn't created yet, call the
' Handle property to force it to be created
For Each ctrl As Control In Me.Controls
While Not ctrl.IsHandleCreated
Dim tmp = ctrl.Handle
tmp = Nothing
End While ' Not ctrl.IsHandleCreated
Next ' ctrl As Control In Me.Controls
It's rather ghetto, but it may help you here (If you still need the help)

Answer 3

I think the issue here has nothing to do with invoking, but references. Following this pseudocode...
Dim A As New Form1
A.Show()
''Spawn background thread with a reference to A
A.Dispose()
Dim B As New Form1
B.Show()
The thread is attempting to refer to the first instance of Form1 above which is disposed and will always stay that way.
If you want the thread to be able to update any form then you need to give the thread a (synchronised) way to refer to the form...
Public Class Worker
Private Target As Form1
Private TargetLock As New Object
Public Sub SetTargetForm(Frm as Form1)
SyncLock TargetLock
Target = Frm
End SyncLock
End Sub
Public Sub DoWork() ''The worker thread method
''Do work as usual then...
SyncLock TargetLock
If Target IsNot Nothing AndAlso Target.IsHandleCreated Then
If Target.InvokeRequired
Target.Invoke(...)
Else
...
End If
End If
End SyncLock
End Sub
End Class
This way, when a new form is available, you can inform the worker thread using SetTargetForm() and it will update the appropriate one.
Of course, you'd be better off refactoring the "Update UI" checks and invoke calls into a different method for simplicity/maintainability but you get the point.
Note that I haven't got an IDE to hand so there may be typos.
One final point... I'd question the value of disposing a form for memory management purposes. Controls are fairly lightweight in terms of memory and it's far more common for an object used by the form to be a memory hog than the form itself. Are you sure you're getting a real benefit for this added complexity?