DropDownList Focus is not working in ASP.Net

Try this

<asp:TextBox runat="server" />  
<asp:DropDownList ID = "DropDownList1" runat="server">
    <asp:ListItem Text="text1" />
    <asp:ListItem Text="text2" />
</asp:DropDownList>
<script type = "text/javascript">
    window.onload = function () {
        document.getElementById("<%=DropDownList1.ClientID %>").focus();
    };
</script>