post back dropdowlist using razor (.net 4.0)

If I understand the questions correctly you will need to use ajax to do this. In MVC controls don't have a postback method like they do in web forms. You will need to hook it up with javascript.

Here is an example of how to get that working.

<script type="text/javascript">
$("#ddl").change(function(e) {
   // Code to update the page or do what ever you need to do when the select changes.  
});
</script>