vb.net / C# External handler - vb.net

Question

I have a form used to display options about processes.
When options are applyed :
frmOptions
For Each ltvi As ListViewItem In ltvProcesses.CheckedItems
Dim proc As Process = CType(ltvi.Tag, Process)
targeted_processes.Add(proc)
AddHandler proc.Exited, AddressOf frmAET.a_target_process_has_been_exited
proc.EnableRaisingEvents = True
Next
And in a tools module :
Public Sub a_target_process_has_been_exited(sender As Object, e As EventArgs)
frmAET.btnStatus.ForeColor = Color.Red
msgbox("OK")
End Sub
And... the messagebox displays its message but the color doesn't change.
After some tries, the problem is when a_target_process_has_been_exited is actived by the handler.
If I do this (Button1 belongs to frmAET, like btnStatus) :
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
a_target_process_has_been_exited()
End Sub
It works ! But not when I really want (when a process is ended).
So, the problem is when the sub is called by the process end event.
And when I try to specify this (maybe a frmAET's sub can modify its controls) :
AddHandler leproc.Exited, AddressOf frmAET.a_target_process_has_been_exited
Error : Reference to a non-shared member requires an objet reference
Could you help me ?

Answer 1

Your AddHandler seems to use AddressOf frmAET.a_target_process_has_been_exited, that means method in frmAET form itself. Not tools module as you stated.
Let's consider your frmOptions is correct and frmAET is containing this (with removed explicit reference to frmAET, since it's local)
Public Sub a_target_process_has_been_exited(sender As Object, e As EventArgs)
btnStatus.ForeColor = Color.Red
MsgBox("OK")
End Sub
As comments already explained, your event handler is called in another thread and you need to sync yourself to main UI thread. For example like this:
Public Sub a_target_process_has_been_exited(sender As Object, e As EventArgs)
Me.BeginInvoke(Sub() HandleProcessExit())
End Sub
Public Sub HandleProcessExit
btnStatus.ForeColor = Color.Red
MsgBox("OK")
End Sub
This version will block main UI thread until you click on the MsgBox button.
You should add some Try/Catch block. Exception in another threads are difficult to detect otherwise.
This code depends on implicit form instances that VB.NET creates for you. I expect your frmAET is actually My.Forms.frmAET instance to make this work.