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The OnClientDropDownOpening client-side event occurs before the drop-down list opens.

The event handler receives two parameters:

  1. The instance of the combobox firing the event.

  2. An eventArgs parameter containing the following methods:

  • set_cancel lets you prevent the drop-down from opening.

  • get_domEvent() returns the DOM event object of the drop-down opening.

The example below shows how to prevent the drop-down list from opening when there are no items to be displayed:

<script language="javascript" type="text/javascript">
    function OnClientDropDownOpening(sender, eventArgs) {
        if (sender.get_items().get_count() == 0) 

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