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The OnClientCheckAllChecking client-side occurs before the check all items check box is checked. This event is added in 2013 Q3 release (Telerik.Web.UI.dll v.2013.3.1015).

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.

  • get_checked() returns the value of the Check All check box before it is checked.

The example below shows how to cancel the event if the check all check box is unchecked:

<telerik:RadComboBox RenderMode="Lightweight" ID="RadComboBox1" runat="server" OnClientCheckAllChecking="OnClientCheckAllChecking"
    CheckBoxes="true" EnableCheckAllItemsCheckBox="true">
    <telerik:RadComboBoxItem Text="Item1" />
    <telerik:RadComboBoxItem Text="Item2" />
    <telerik:RadComboBoxItem Text="Item3" />
function OnClientCheckAllChecking(sender, args) {
    if (args.get_checked()) {
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