<%@ Page Language="C#" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<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>
https://docs.microsoft.com/zh-cn/dotnet/api/system.web.ui.attributecollection.add?redirectedfrom=MSDN&view=netframework-4.7.2#System_Web_UI_AttributeCollection_Add_System_String_System_String_