Question

I have an Entity with the following fields
GUID TaskId,
GUID SubTaskId,
DateTime Timestamp

The TaskId may be one-to-many with the SubTaskId. It’s like a composite key almost.

I would like to write an expression or expressions resulting in the newest among each TaskId
i.e.

TaskId SubTaskId Timestamp
1 A 2010-11-22 15:48:49.727
1 B 2010-11-22 16:24:51.117
2 C 2010-11-15 11:49:05.717
2 D 2010-11-15 14:02:11.467

Would result in a sequence containing only:
TaskId SubTaskId Timestamp
1 B 2010-11-22 16:24:51.117
2 D 2010-11-15 14:02:11.467

This expression works just fine for showing all TaskId’s and TimeStamps

var q1 = 
    from req in myEntity
    orderby req.TaskId
    select new
    {
        req.TaskId,
        req.SubTaskId,
        req.Timestamp
    };

but I would like to show only the latest Timestamp for each TaskId. All my attempts at adding a max to the expression fail. The posting (http://stackoverflow.com/questions/2034800/how-do-you-find-the-group-wise-max-in-linq) seemed close, but I couldn’t translate it into something working. Perhaps a foreach is needed.

Answer 1

How about this?

var q1 = 
    from req in myEntity
    group req by req.TaskId into g
    let topReq = g.OrderByDescending(r => r.Timestamp).FirstOrDefault()
    select new
    {
        topReq.TaskId,
        topReq.SubTaskId,
        topReq.Timestamp
    };