Question

I have the following

IList<Project> projects = (from update in dbContext.TicketUpdates
               where update.TicketUpdatesEmployeeID == Profile.EmployeeID
               orderby update.TicketUpdatesEndDate descending
               select update.Ticket.Project).Distinct().ToList();

I understand that Distinct() doesn’t guarantee order, but I can’t reorder it afterwards because the projection loses the field I”m using to order. How can I work around this?

Answer 1

I think your best bet is to post process to preserve ordering:

var projects = (from update in dbContext.TicketUpdates
           where update.TicketUpdatesEmployeeID == Profile.EmployeeID
           orderby update.TicketUpdatesEndDate descending
           select update.Ticket.Project);
var seen = new HashSet<Project>();
foreach (var project in projects)
{
    if (seen.Add(project))
    {
        // A distinct project
    }
}

Or you could abuse GroupBy:

var projects = dbContext.TicketUpdates
    .Where(uu => uu.TicketUpdatesEmployeeID == Profile.EmployeeID)
    .GroupBy(uu => uu.Ticket.Project)
    .OrderByDescending(gg => gg.Max(uu => uu.TicketUpdatesEndDate))
    .Select(gg => gg.Key);

By using GroupBy across uu.Ticket.Project each update will be grouped by their associated project. If you have 10 projects and 30 updates spread amongst them, you’ll have 10 groups–one per project–in the output of that stage. Next, we order the groups by their newest end date which preserves the descending order you were looking for. Finally, we select the key from each IGrouping which is the project.