javascript runat server
<script runat="server">
protected void SubmitBtn_Click(object sender, EventArgs e)
{
firstselect.Attributes.Add("Multiple", "True");
}
</script>
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title>AttributeCollection Example</title>
</head>
<body>
<form id="form1" runat="server">
<div>
Make a selection:
<select id="firstselect"
runat="server">
<option>This</option>
<option>That</option>
<option>Other</option>
</select>
<br/><br/>
<input type="submit"
id="submitbtn"
value="modify attribute"
onserverclick="SubmitBtn_Click"
runat="server"/>
</div>
</form>
</body>
</html>
