Multithread unable to get updated textbox value.

I encountered problem getting the updated textbox value after i start new thread for my application, any help will be much appreciated. Sequence as follow:
Private Sub Form1_Load1(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
   textbox1.text = "aaaa"
End Sub
Private Sub Btn1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Btn1.Click
   textbox1.text = "bbbb"
 End Sub
Private Sub StartBtn_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles StartBtn.Click
   Timer1.Enabled = True
End Sub

Private Delegate Sub SBODelegate(ByVal i As Integer, ByVal i As Integer)
Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
   Dim del As SBODelegate = New SBODelegate(AddressOf SBOThread)
   Dim res As IAsyncResult = del.BeginInvoke(1, 2, Nothing, Nothing)
End Sub
Private Sub SBOThread(ByVal x As Integer, ByVal y As Integer)
   SBO.SBOGetTextbox()
End Sub
Public Class SBO
   Public Shared Sub SBOGetTextbox()
      Messagebox.show(textbox1.text)
   End Sub
End Class
Can anybody tell me why the result from the message box is "aaaa", and not "bbbb"?