Question

Here is a code sample, you will need to place a ListBox called list_of_items onto the form, to reproduce the issue I’m having:

Imports System.Threading.Tasks

Public Class Form1
  Dim _dt As DataTable

  Private Sub Form1_Load() Handles MyBase.Load
    _dt = New DataTable
    With _dt.Columns
      .Add("key")
      .Add("value")
    End With
    With list_of_items
      .ValueMember = "key"
      .DisplayMember = "value"
      .DataSource = _dt
    End With
    Dim addItemsTask As New Task(AddressOf AddThreeItems)
    addItemsTask.Start() 'does not add anything when done
    'AddThreeItems() #doing this instead works!
  End Sub

  Private Sub AddThreeItems()
    Threading.Thread.Sleep(2000)
    With _dt.Rows
      .Add({"1", "One"})
      .Add({"2", "Two"})
      .Add({"3", "Three"})
    End With
    Me.Invoke(Sub() Me.Text = "Separate thread is done")
  End Sub
End Class

Problem is that rows do get added physically, so DataTable.Rows.Count increases, but visually nothing happens. I tried calling Refresh, resetting DataSource to Nothing and back – it does not help. If I switch this to single thread processing, rows get added just fine with the illustrated approach. What could be the issue?

Answer 1

I was playing around Me.Invoke and found that if I add a dummy record synchronously using Invoke, all records I added previously actually get added. Then I just need to remove this dummy record from a DataTable. If you have a better solution or a more elegant workaround, or you can explain why it works like this, please feel free to post it as an answer. Here is how the code looks now:

Private Sub AddThreeItems()
  Threading.Thread.Sleep(2000)
  With _dt.Rows
    .Add({"1", "One"})
    .Add({"2", "Two"})
    .Add({"3", "Three"})
  End With
  Me.Invoke(Sub()
              _dt.Rows.Add()
              _dt.Rows.RemoveAt(_dt.Rows.Count - 1)
              Me.Text = "Separate thread is done"
            End Sub)
End Sub