VB.Net Updating UI from background thread - vb.net

Question

I’m working on a scheduler like project using VB.Net, the application start from “Sub Main” with Application.Run(), all program codes are handler in a class, which is created and started here,
Public Sub Main()
m_App = New myApp
m_App.Start()
Application.Run()
End Sub
Inside the myApp, there has a timer to control the execution of task, and it will start a thread for each task, when the task complete, we try to display the alert window if there has error detected. We have tested two different way for communization between the execute thread and the main thread in order to display the alert window (frmAlert):
1) by adding pulic event in task object, then addhandler to the function in main thread
2) use delegate to notify the main thread
However, the alert window cannot be shown and there has no error reported. After debuging with the IDE, it was found that the alert windows has successful displayed, but it will closed when the task thread is completed.
Here is a simplified task class (testing with two communization methods),
Public Class myProcess
Public Event NotifyEvent()
Public Delegate Sub NotifyDelegate()
Private m_NotifyDelegate As NotifyDelegate
Public Sub SetNotify(ByVal NotifyDelegate As NotifyDelegate)
m_NotifyDelegate = NotifyDelegate
End Sub
Public Sub Execute()
System.Threading.Thread.Sleep(2000)
RaiseEvent NotifyEvent()
If m_NotifyDelegate IsNot Nothing Then m_NotifyDelegate()
End Sub
End Class
And the main application class
Imports System.Threading
Public Class myApp
Private WithEvents _Timer As New Windows.Forms.Timer
Private m_Process As New myProcess
Public Sub Start()
AddHandler m_Process.NotifyEvent, AddressOf Me.NotifyEvent
m_Process.SetNotify(AddressOf NotifyDelegate)
ProcessTasks()
End Sub
Private Sub Timer_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles _Timer.Tick
ProcessTasks()
End Sub
Public Sub ProcessTasks()
_Timer.Enabled = False
'
Dim m_Thread = New Thread(AddressOf m_Process.Execute)
m_Thread.Start()
'
_Timer.Interval = 30000
_Timer.Enabled = True
End Sub
Public Sub NotifyEvent()
frmAlert.Show()
End Sub
Public Sub NotifyDelegate()
frmAlert.Show()
End Sub
End Class
It was found that the frmAlert is shown by using either NotifyEvent or NotifyDelegate, but it is closed immediately when the Execute is finished.
May I know how we can popup an alert window from the execution thread which can stay in the screen until user close it?
Thanks in advance!

Answer 1

You need to ensure that your main thread does not terminate if you want it to do anything when a sub-thread raises and event.
Public Sub Start()
AddHandler m_Process.NotifyEvent, AddressOf Me.NotifyEvent
m_Process.SetNotify(AddressOf NotifyDelegate)
ProcessTasks()
Do While True 'You might want to add a boolean condition here to instruct the main program to terminate when you want it to.
System.Threading.Thread.Sleep(200)
Loop
End Sub
This will prevent the main thread (program) to end, and thus will be available to handle any events raised by the sub-threads. Note: Your class lacks a termination condition.