how to fire onchange event

hi every body. i m writing  a desktop  programme which is developed in .net c# and framework 2.0. i want to use a web site in my application automaticaly. now i can fill related textboxes and select right options on comboboxes on web site through my desktop application. here is my problem. a combobox has a javascript on its 'onchange event'. when i selected my option on this combobox, onchange event did not fire.

mshtml.HTMLSelectElement combo2= (mshtml.HTMLSelectElement)mshtml.HTMLSelectElementClass)doc.all.item("select_city",System.Reflection.Missing.Value);
           
            mshtml.HTMLSelectElement combo1 = (mshtml.HTMLSelectElement)doc.all.item("select_country", System.Reflection.Missing.Value);
 
combo1.selectedIndex=comboboxCountry.SelectedIndex;

combo2.selectedIndex=comboboxCity.SelectedIndex;

i want onchange event to fire when i choose my selection on combo2 . but it doesnt. what should i  do?

i know that i should use fire event method. but i could not find any  detailed example. thanks a lot.

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software engineer