The OnClientItemChecking client-side event occurs when the user selects a checkbox using mouse or keyboard. The checked state returned by the item's get_checked() function has not changed to reflect the new value. The event can be cancelled.

The event handler receives two parameters:

  1. The instance of the listbox firing the event

  2. An event arguments parameter containing the following methods:

  • get_item() - returns the checked item

  • set_cancel() - lets you cancel the event and prevent changing the checked state of the item.

  • get_domEvent() - returns the DOM event object

The example below shows how to check an item after the user clicks OK in the confirmation dialog:

function OnClientItemCheckingHandler(sender, eventArgs) {
    var item = eventArgs.get_item();
    var message = "Are you sure you want to ";
    message += item.get_checked() ? "uncheck" : "check";
    message += " the item " + item.get_text();
    if (!confirm(message))

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