CheckAllCheck

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">
    <Items>
        <telerik:RadComboBoxItem Text="Item1" />
        <telerik:RadComboBoxItem Text="Item2" />
        <telerik:RadComboBoxItem Text="Item3" />
    </Items>
</telerik:RadComboBox>
<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.";
    }
    else
    {
        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."
    Else
        Label1.Text = "All items are unchecked"
    End If
End Sub