Cannot Open Form Created on Separate Thread After Closing

Question

My application communicates with a device that has several sensors.  As the sensors collect data, they send messages over the com port.  I have written a class to communicate with the device.  As the messages come in and are processed, the
class raises events that the application responds to.
The main window of the application handles the communication with the device and displays several statistics based on the data collected.  When the user presses a button on the device, a specific event is raised.  The main window create a separate
thread and opens a child window.  When the child window is open, the user opens a valve to dispense the product.  As the product is dispensed, a flow meter connected to the device measures the volume of product dispensed.  The flow meter generates
messages to indicate the volume dispensed.  I need to be able to send messages from the main window to the child window so that the child window displays the volume.  When the user is done, they close the valve dispensing the product and press the
"End" button on the child window.  The child window then updates several variables on the main window, makes a couple of database calls to record how much product was dispensed and by whom and then closes.
I need to run the child window using a separate thread as both windows need to be able to process commands.  If only one window has control the program doesn't work at all.  I have it figured out so that everything is working.  I can open
the child window, dispense product, se the amount of product dispensed in the child window (the main window processes commands from the device and updates the label on the child window using a delegate), closes the window (using Me.Close()) and updates the
main display with the updated data.  The problem is that when a user goes to dispense product a second time, I get the following error:
  A first chance exception of type 'System.ObjectDisposedException' occurred in System.Windows.Forms.dll
  Additional information: Cannot access a disposed object.
I thought that maybe I could hide the window (change Me.Close() to Me.Hide) and then just show it.  When I do that I get this error:
  A first chance exception of type 'System.InvalidOperationException' occurred in System.Windows.Forms.dll
  Additional information: Cross-thread operation not valid: Control 'frmPour' accessed from a thread other than the thread it was created on.
Both of these errors make sense to me, I just can't figure out how to make it work.
First I have to declare the child window as a global variable as I need to access the window from a couple of the event handlers in the main form.
Public frmMeasure As New frmPour
When the user presses the button on the device to dispense the product, the event handler executes this code to open the child window.
Private Sub StartPour(sAuthName As String, sAuthToken As String, iStatus As Integer) Handles Device.Pour
Dim th As System.Threading.Thread = New Threading.Thread(AddressOf Me.OpenDispenseWindow)
th.SetApartmentState(ApartmentState.STA)
th.Start()
End If
End Sub
Which executes this code:
Public Sub OpenDispenseWindow()
frmMeasure.sNameProperty = sCurrentUserName
frmMeasure.sAuthTokenIDProperty = sUserToken
Application.Run(frmMeasure)
bAuthenticated = False
bPouring = False
dSessionVolume += GetTapConversion(sCurrentValve) * iFinalTick
UpdateDisplayDelegate(iValveID)
End Sub
It doesn't matter if I use Me.Close() or Me.Hide(), both methods fail on the Application.Run(frmMeasure) line with the errors shown above. 
For the Me.Close() method, my thinking is that the global frmMeasure object is getting disposed when I close the child window.  Is there any way that I can re-instantiate it when I go to display the window again?
For the Me.Hide method, is there any way that I can track the thread that created it in the main window and when I go to call it a second time, detect that it is already available and just Show() it?
Any hints, tips or suggestions are appreciated.
Thanks.
John
John

Answer 1

To be honest, I have only grasped < 100% of your message in detail, but...: Windows that have a parent<->child relation must be running in the same thread. In addition, after closing a modeless window, you must not use it anymore. Instead, create
a new instance.
What happens if you do not create a new thread but instead open the child in the same thread (which is obligatory)? You wrote it doesn't work, but I don't know why?
"First I have to declare the child window as a global variable".
How do you define "global"? Normally this is a variable in a Module declared with the scope Public or Friend. But I guess you mean a field of the Form (a variable at class level in the Form).
"I need to be able to send messages from the main window to the child window so that the child window displays the volume."
Why does the main window has to send the messages? Can't the child window handle the device's messages itself?
"I need to run the child window using a separate thread as both windows need to be able to process commands."
Process commands from the device, or commands from the user operating the Forms?
Armin