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The OnClientCheckAllChecking client-side event occurs when the user clicks on the CheckAll item. The checked state of this item can be inspected from the returned DOM event object - args.get_domEvent().target.checked. The event can be canceled.

The event handler receives two parameters:

  1. The instance of the listbox firing the event

  2. An event arguments parameter containing the following methods:

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

  • get_domEvent() - returns the DOM event object

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