What are you trying to accomplish?

i am trying create xml file of a sales order with all the line on a button click and also change line level data like price , item code etc for all the lines of a sales order

Move your button so it is associated with the header object (not lines), and set it to run from the server (not client) so you don't need to pass through all the data references (accessing business objects directly).  Then create a lines object and loop through them.

set oLines = oSession.AsObject(oBusObj.Lines)

retVal = oLines.MoveFirst()

do until oLines.EoF

' do stuff…

retVal = oLines.MoveNext()

loop 

Thanks a lot Kevin M

Now i am able to loop through lines but still i am not able to read data , i am not sure if i am doing something wrong , i tried
oLines.nGetKey(),
oLines.nGetKeyvalue("LineKey$"),
oLine.nGetResultSets("QuantityOrdered$", "UnitOfMeasure$", Quantity, UOM,"", "", "")

can you please help me out

Take a look at KB article 43033.
retVal = oLines.GetValue(column$, sVariable)