For Each ctrl As Control In Me.Form.Controls
If TypeOf ctrl Is ContentPlaceHolder Then
Dim chp As ContentPlaceHolder = DirectCast(ctrl, System.Web.UI.WebControls.ContentPlaceHolder)
For Each ctrl2 As Control In chp.Controls
If TypeOf ctrl2 Is LinkButton Then
Dim tb As TextBox = DirectCast(ctrl2, System.Web.UI.WebControls.LinkButton)
tb.Text = "special text"
End If
Next
End If
Next