Public Class Form1
  Dim collection As New AutoCompleteStringCollection
  Private ReadOnly arrayCollection() As String = {"a"}

  Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load

  End Sub

  Public Sub New()

    InitializeComponent()
    collection.AddRange(New String() {"apple", "aero", "banana"})
    TextBox1.AutoCompleteCustomSource = collection
    ReDim arrayCollection(collection.Count - 1)
    collection.CopyTo(arrayCollection, 0)
  End Sub
  ''' <summary>
  ''' When release the keys, plz start a background thread to handle the problem
  ''' </summary>
  Private Sub TextBox1_KeyUp(sender As Object, e As KeyEventArgs) Handles TextBox1.KeyUp
    Dim act As New Action(Sub()
                 'Check whether there are any values inside the collection or not
                 If (TextBox1.Text = "") OrElse (arrayCollection.Any(Function(s)
                                             Return s.StartsWith(TextBox1.Text)
                                           End Function)) Then

                   Label1.BeginInvoke(New MethodInvoker(Sub()
                                        Label1.Text = String.Empty
                                      End Sub))
                 Else
                   Label1.BeginInvoke(New MethodInvoker(Sub()
                                        Label1.Text = "Not found"
                                      End Sub))
                 End If

               End Sub)
    act.BeginInvoke(Nothing, Nothing)
  End Sub
End Class