动态给服务器端控件增添属性

<asp:DropDownList ID="DropDownList1" runat="server">
    <asp:ListItem Value="20">张三</asp:ListItem>
    <asp:ListItem Value="21">李四</asp:ListItem>
    <asp:ListItem Value="22">王五</asp:ListItem>
</asp:DropDownList>
<script type="text/javascript">
function showvalue(obj)
{
    alert(obj.value);
}
</script>

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        DropDownList1.Attributes.Add("onchange", "showvalue(this);");
    }
}