Updating UDF in SO Line

i am trying to update a UDF in line level using script on a button click

Dim count
Set fso = CreateObject("Scripting.FileSystemObject")
Set dict = CreateObject("Scripting.Dictionary")
If (fso.FileExists("C:\XIDT\line.txt")) Then
Set file = fso.OpenTextFile ("C:\XIDT\line.txt", 1)
set oLines = oSession.AsObject(oBusObj.Lines)
retVal = oLines.MoveFirst()
set Salesobj = oSession.AsObject(oSession.GetObject("SO_SalesOrder_BUS"))
'msgbox(SO_SalesOrder_bus_SalesOrderNo)
Salesobj.SetKey(SO_SalesOrder_bus_SalesOrderNo)
Do Until file.AtEndOfStream
count =""
line = file.Readline
count=count+line
retVal =oLines.SetValue("UDF_TEST$",count)
retVal =oLines.Write()
retVal = oLines.MoveNext()
retVal = oSession.AsObject(oSession.UI).MessageBox("",count)
Loop
retVal = Salesobj.Write()
file.Close
else
retVal = oSession.AsObject(oSession.UI).MessageBox("","bla")
end if

its updating all but the last line can any one help

thanks