在事件处理程序无效的情况下调用代码

I have a web page that consists of two dropdown boxes and a gridview. I have created a web service that collects the data for the drop down boxes and I have used a ajax call to populate them. The issue I am having is when I try to add a OnSelectedIndexChanged event handler, it does not seem to be working correctly. I have looked around and I have tried a few things. I have set EnableEventValidation to false (I presume this fixed an issue related to my ddl being populated dynamically in js). I have also enabled AutoPostBack on my ddl.I have a web page that consists of two dropdown