用JS获取DropDownList选中得值

HTML:

<td style="width: 20%">
<asp:DropDownList ID="DropdownList1" runat="server" AutoPostBack="true" />
</td>

<input type="button" id="btnSubmit" name="btnSubmit" value="保存" οnclick="getvalue();"/>

JS:

<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>

其他网上给出的：

<script type="text/javascript" language="javascript">
function SearchChange()
{
var ddl = document.getElementById("DropDownList1")
var index = ddl.selectedIndex;

var Value = ddl.options[index].value;
var Text = ddl.options[index].text;

alert(Value);
}
</script>

方式获取的ddl 为null；