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The CheckAllCheck event occurs when the check all items check box of the RadComboBox is changed.

The CheckAllCheck event does not fire unless the AutoPostBack property is True .

The CheckAllCheck receives one argument which indicates whether the check all check box is checked or unchecked.

<telerik:RadComboBox RenderMode="Lightweight" ID="RadComboBox1" runat="server" OnCheckAllCheck="RadComboBox1_CheckAllCheck"
    EnableCheckAllItemsCheckBox="true" AutoPostBack="true" CheckBoxes="true">
        <telerik:RadComboBoxItem Text="Item1" />
        <telerik:RadComboBoxItem Text="Item2" />
        <telerik:RadComboBoxItem Text="Item3" />
<asp:Label ID="Label1" runat="server" />
protected void RadComboBox1_CheckAllCheck(object sender, RadComboBoxCheckAllCheckEventArgs e)
    bool allChecked = e.CheckAllChecked;
    if (allChecked)
        Label1.Text = "All items are checked.";
        Label1.Text = "All items are unchecked";
Protected Sub RadComboBox1_CheckAllCheck(sender As Object, e As RadComboBoxCheckAllCheckEventArgs)
    Dim allChecked As Boolean = e.CheckAllChecked
    If allChecked Then
        Label1.Text = "All items are checked."
        Label1.Text = "All items are unchecked"
    End If
End Sub
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