Using TASK class in VB.NET async

Question

This is my first post here, so i hope to be very explicit about my problem.

Currently i'm working with a database, and at some point, i'm having some problems with the latency because of the data amont.

What i want is, when i press a button in the MainForm to call separate Task that takes a method that populates a comboBox in another form, and then display that form.

Here is the code :

Private Sub PopulateCmb()
    For Each value As Integer In servRefrence.PopulateID_Masini
        Form_Insert.ComboBox1.Items.Add(value)
    Next
    <<This is taking 3-4 seconds>>
End Sub

Private Async Sub BtnInsert_Click(sender As Object, e As EventArgs) Handles BtnInsert.Click

    Dim task As Task = New Task(New Action(AddressOf PopulateCmb))
    task.Start()
    LblInfo.Text = "Please Wait"
    Await task

    LblInfo.Text = "Idle"
    Form_Insert.Show()

End Sub

This method : servRefrence.PopulateID_Masini is returning a list of integers that comes from the webserver...

The problem with this code is that when the task is finished, the combobox within the Form_Insert comes back empty!

What am i missing??...I think is maybe because you cannot make use of a task within the mainForm(for ex.), and try to "modify" some data in a second form...but i'm not sure...

==I am using Visual Studio 2012== Template : Visual Basic

Please help!

Thank You!!!

You shouldn't ever use the Task constructor.

Stephen Cleary
– Stephen Cleary

2017-08-02 13:23:00 +00:00
Commented Aug 2, 2017 at 13:23 — Stephen Cleary
– Stephen Cleary, Commented Aug 2, 2017 at 13:23

Jan Muncinsky · Accepted Answer · 2017-08-02 08:43:35Z

2

Problem is that you are populating the comboBox in non-UI thread. I guess that latency is caused by service call, so you should populate combobox after task await in UI thread.

Dim task = Task.Factory.StartNew(Of List(Of Integer))(
           Function() As List(Of Integer)
                 Return servRefrence.PopulateID_Masini
           End Function)

Dim result = Await task

For Each value As Integer In result
    Form_Insert.ComboBox1.Items.Add(value)
Next

answered Aug 2, 2017 at 8:43

Jan Muncinsky

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1 Comment

Stephen Cleary Over a year ago

Yes. It would be even better to use Task.Run instead of Task.Factory.StartNew.

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