Validate CheckBoxList Server Side for atleast One Item Selected using ASP.Net

This way

  <asp:CheckBoxList ID="CheckBoxList1" runat="server" RepeatColumns="3">
            <asp:ListItem>1st</asp:ListItem>
            <asp:ListItem>2nd</asp:ListItem>
            <asp:ListItem>3rd</asp:ListItem>
        </asp:CheckBoxList>
        <asp:Button ID="Button1" runat="server" Text="Submit" 
            onclick="Button1_Click1" />

    protected void Button1_Click1(object sender, EventArgs e)
    {
        bool isChecked = false;
        foreach (ListItem item in CheckBoxList1.Items)
        {
            if (item.Selected)
            {
                isChecked = true;
                break;
            }
        }
        if (!isChecked)
        {
            ClientScript.RegisterStartupScript(GetType(), "alert", "alert('Please select atleast one item!')", true);
            return;
        }
    }