Can not bind child table in Combo box or list Box

Hi
I'm using vb.net with web service to develop a simple pocket pc app. The
web service is handling to generate a dataset with parent table and a child
table with a relation attached. My ppc app is able to get the return dataset
from the web service and is ok to bind the parent to the parent combo box
like parentcombo.datasource = ds.parentable(0) and parentcombo.displaymember
as ds.parentable(0).item("id"). BUT I can't bind the child (relation) to my
child list box which is childlistbox.datasource = ds.parentable(0)
childlistbox.displaymember = "therelationname.childfield".. I have tested
the code at normal windows form and is working fine as when I Chance the
combox box value the child list view show the related value, But it doesn't
work in ppc app.

Please help.......
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Thanks & Best Regards
S.S.Chon