adding a new row with a dropdownlist to a gridview dynamically

I am looking for the code to added a new row with a dropdownlist in a gridview when a button is clicked loading a datasource for the dropdownlist provide the ability to use to gridview row count to process each ddl in each row.  

Example.

GridView
       Row  >   DropDownList
End of Grid View

Now give the ability to add a Row with a DropdownList to the GridView

Now using the Row Count of the GridView display selected values of the dropdownlist.