The problem is solved.  It was not a matter of the code being wrong or 
accessing the wrong variable.   What was actually the matter was that I 
was rebinding the table to the datagrid on every page load (even postback).

I put the datasource and databind commands into a conditional

if ( !IsPostBack )
{
   // set datasource...
   // databind...
}

and now everything works as expected.

David Garner