asp.net 实现下拉框只读功能

HTML>
HEAD>
TITLE>下拉框模拟只读/TITLE>
script type="text/javascript">
//根据下拉框ID设置下拉框只读
function setReadOnly(obj_id){
var obj = document.getElementById(obj_id);
obj.onmouseover = function(){
obj.setCapture();
}
obj.onmouseout = function(){
obj.releaseCapture();
}
obj.onfocus = function(){
obj.blur();
}
obj.onbeforeactivate = function(){
return false;
}
}
/script>
/HEAD>
BODY>
span id="span_select">
select id="id_select">
option value="1">AAAAAAAAA/option>
option value="2">BBBBBBBBB/option>
option value="3">CCCCCCCCC/option>
option value="4">DDDDDDDDD/option>
/select>
/span>
script type="text/javascript">
setReadOnly("span_select");
/script>
!-- 直接设置下拉框只读 -->
select onbeforeactivate="return false" onfocus="this.blur()" onmouseover="this.setCapture()" onmouseout="this.releaseCapture()">
/BODY>
/HTML>

this.ddlSysCategory.Attributes.Add("onfocus","this.blur();");
this.ddlSysCategory.Attributes.Add("onbeforeactivate","return false");
this.ddlSysCategory.Attributes.Add("onmouseover","this.setCapture()");
this.ddlSysCategory.Attributes.Add("onmouseout","this.releaseCapture()");