custom binding: code expression

[Guest]

hey all,
i have a bound drop-down list where the contents change all the time. i'm
having trouble with the SelectedValue binding because there's a scenario
where the value could be null, in which case I want to default the
dropdownlist to the selectedIndex 0.

Is there a way to make my SelectedValue Binding Custom Expression
conditional and if it's null just pick the first index?

thanks,
rodchar