Can someone show me how to indicate which columns I would like returned at run-time from a LINQ To SQL statement?

I am allowing the user to select items in a checkboxlist representing the columns they would like displayed in a gridview that is bound to the results of a L2S query.

I am able to dynamically generate the WHERE clause but am unable to do the same with the SELECT piece. Here is a sample:

var query = from log in context.Logs select log;
                query = query.Where(Log => Log.Timestamp > CustomReport.ReportDateStart);
                query = query.Where(Log => Log.Timestamp < CustomReport.ReportDateEnd);
                query = query.Where(Log => Log.ProcessName == CustomReport.ProcessName);

                foreach (Pair filter in CustomReport.ExtColsToFilter)
                {
                    sExtFilters = "<key>" + filter.First + "</key><value>" + filter.Second + "</value>";
                    query = query.Where(Log => Log.FormattedMessage.Contains(sExtFilters));
                }