asp.net实现下拉框只读功能

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()");