用JS获取DropDownList选中得值

<asp:DropDownList ID="DropdownList1" runat="server" AutoPostBack="true" />

<script type="text/JavaScript">
function getvalue(){

var select1 = document.all.<%= DropdownList1.ClientID %>;
var selectvalue = select1.options[select1.selectedIndex].value;
var selectText= select1.options[select1.selectedIndex].text;
alert(selectvalue+"  "+selectText);
}
</script>