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>Since the user can only be using one control at a time, why have the cities update each time the state changes? If they arrow down through the list of states, won't that be constantly changing the cities? Why not do it in the lost focus since the state combo would have to lose focus before the cities combo can get focus, anyway?
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Michelle,

Because the user cna see the city combo and it may be showing an invalid city. I prefer that the user sees the city combo respond to their selection in the state combo. I want it to refresh with the proper cities as the user arrows through the states. It makes the whole app seem more responsive. Other may prefer to do it in the lostfocus, but then this could appear:

 City: [Boston   \/]    State: [New York   \/}]

With my approach, as soon as New York is selected as the state the city list would be refreshed to only NY cities and Boston would disappear from the combo.