Imports System.Threading

Public Class Form1
    Private Sub Button1_Click() Handles Button1.Click
        ' Display a messagebox with the UI thread ID
        MessageBox.Show($"Long task starting. UI Thread ID {Thread.CurrentThread.ManagedThreadId}")
        ' create a new thread to perform the long running task
        Dim t As New Thread(AddressOf DoWork)
        t.IsBackground = True
        t.Start()
    End Sub

    Private Sub DoWork()
        MessageBox.Show($"Long task starting. Secondary Thread ID {Thread.CurrentThread.ManagedThreadId}")
        ' Pretend to perform a long running task off the UI thread. This will stop the UI from freezing.
        Thread.Sleep(5000)
        ' Pretend long task finished.
        ' To access the forms UI thread we must invoke back to the UI thread.
        Me.Invoke(Sub()
                      ' now we are back on the UI thread.
                      MessageBox.Show($"Long task starting. UI Thread ID {Thread.CurrentThread.ManagedThreadId}")
                      Me.Text = $"Long task starting. UI Thread ID {Thread.CurrentThread.ManagedThreadId}"
                  End Sub)
    End Sub
End Class