how to check the checkbox inside a grid

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false">
    <Columns>
        <asp:TemplateField>
            <ItemTemplate>
                <asp:CheckBox ID="CheckBox1" runat="server" Text='<%# Eval("Id") %>' />
            </ItemTemplate>
        </asp:TemplateField>
        <asp:BoundField DataField="Name" HeaderText="Name" />
        <asp:BoundField DataField="Country" HeaderText="Country" />
    </Columns>
</asp:GridView>
<asp:CustomValidator ID="CustomValidator1" runat="server" ErrorMessage="Please select at least one record." ClientValidationFunction="Validate" ForeColor="Red"></asp:CustomValidator>
<br />
<asp:Button ID="Button1" runat="server" Text="Submit" />

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        DataTable dt = new DataTable();
        dt.Columns.AddRange(new DataColumn[3] { new DataColumn("Id"), new DataColumn("Name"),     new DataColumn("Country") });
        dt.Rows.Add(1, "John Hammond", "United States");
        dt.Rows.Add(2, "Mudassar Khan", "India");
        dt.Rows.Add(3, "Suzanne Mathews", "France");
        dt.Rows.Add(4, "Robert Schidner", "Russia");
        GridView1.DataSource = dt;
        GridView1.DataBind();
    }
}

<script type="text/javascript">
    function Validate(sender, args) {
        var gridView = document.getElementById("<%=GridView1.ClientID %>");
        var checkBoxes = gridView.getElementsByTagName("input");
        for (var i = 0; i < checkBoxes.length; i++) {
            if (checkBoxes[i].type == "checkbox" && checkBoxes[i].checked) {
                args.IsValid = true;
                return;
            }
        }
        args.IsValid = false;
    }
</script>

</script>