How to handle long running tasks in VB.NET forms? - vb.net

Question

I am currently working on a VB.NET form that automatically create Word documents according to an Excel file and a few extra data asked by the form (Project Name, Customer Name, Use SQL, ...).
This procedure works fine and takes approximatelly 1 or 2 minutes to complete.
The issue is that all my script is in ButtonGenerate.Click Handler. So when the Generate button is pressed the form window is bricked and it's impossible to Cancel...
It shouldn't be in a Click handler. Opening a new thread for that long task seems better. But Multithreading isn't very familiar to me.
I tryed launching the script with
ThreadPool.QueueUserWorkItem(...
but my Generate Sub sets labels and update a Progress Bar in the main form, so I doesn't work unless I use
Me.Invoke(New MethodInvoker(Sub()
label.Text = "..."
ProgressBar.Value = 10
' ...
End Sub)
each time I need to update something on the form and I can't even retrieve any new push of a button with that (A cancel button would be nice).
This is basically my code :
Public Class TestFichesAutomation
Private Sub BtnGenerate_Click(sender As Object, e As EventArgs) Handles BtnGenerate.Click
System.Threading.ThreadPool.QueueUserWorkItem(Sub() Generate())
End Sub
Public Sub Generate()
' Check user options, retrieve Excel Data, SQL, Fill in custom classes, create Word docs (~ 1 minute)
End Sub
So How would you handle that script ? Is Threading even a good solution ?

Answer 1

Thanks a lot for your help ^^ and for the useful doc.
My app now open a new thread and uses 2 custom classes to act like buffers :
Private Async Sub Btn_Click(sender As Object, e As EventArgs) Handles Btn.Click
myProgress = New Progress
' a custom class just for the UI with the current task, current SQL connection status and progress value in %
_Options.ProjectName = TextBoxProjectName.Text
_Options.CustomerName = TextBoxCustomerName.Text
...
' Fill in a custom "_Options" private class to act as a buffer between the 2 thread (the user choices)
Loading = New Loading()
Me.Visible = False
Loading.Show() ' Show the Loading window (a ProgressBar and a label : inputLine)
Task.Run(Function() Generate(Progress, _Options))
Me.Visible = True
End Sub
Public Async Function Generate(ByVal myProgress As Progress, ByVal Options As Options) As Task(Of Boolean)
' DO THE LONG JOB and sometimes update the UI :
myProgress.LoadingValue = 50 ' %
myProgress.CurrentTask= "SQL query : " & ...
Me.Invoke(New MethodInvoker(Sub() UpdateLoading()))
' Check if the task has been cancelled ("Cancelled" is changed by a passvalue from the Loading window):
If myProgress.Cancelled = True Then ...
' Continue ...
End Function
Public Shared Sub UpdateLoading()
MyForm.Loading.ProgressBar.Value = myProgress.LoadingValue
MyForm.Loading.inputLine.Text = myProgress.CurrentTask
' ...
End Sub

Answer 2

You should look into using the Async/Await structure
if the work you need to do is CPU bound, i like using Task.Run() doc here
By making your event handler Async and having it Await the work, you prevent the UI from locking up and avoid the use of Invoke in most cases.
ex:
Private Async Sub Btn_Click(sender As Object, e As EventArgs) Handles Btn.Click
Dim Result As Object = Await Task.Run(Function() SomeFunction())
'after the task returned by Task.Run is completed, the sub will continue, thus allowing you to update the UI etc..
End Sub
For progress reporting with Async/Await you might be interested in this