Changing datasource of dropdownlist

[Julie Barnet]

I have a datagrid that includes a dropdownlist.

I have an edit button which allows me to access the dropdownlist as a
label only (I can only get the displayed value).

I need to be able to access the dropdownlist attributes (ie.
datasource). When the user clicks on the edit button, I would like to
limit the list of the dropdownlist to those values that are applicable
to the fields already entered.

Example:
Field A: Canada
Field B: Ontario
Field C: Toronto

I would only like to see field C based on the values entered in Field
A and Field B (only Ontario cities should be available in the list).

This would also have to be done in the selected index changed event of
the dropdownlist. But I should be good with that one.

Thanks in advance
Julie Barnet