dropdownlist in gridview edit template dosen't maintain viewstate?

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i have a template field in a gridview,i modified the edititem template for a
field to represent a dropdownlist instead of the textbox,i need to make set
of server side code when the user changes the select dropdownlist, although i
turned on autopost back and the enable viewstate properties of the ddl the
selected item is not maintained when the page is reloaded and so the server
side code dosn't work correct?

thank you for help