Question

I have the following object

 public partial class Log
{
    public System.Guid Id { get; set; }
    public double Lat { get; set; }
    public double Lng { get; set; }
    public System.DateTime TimeStamp { get; set; }
    public int DeviceId { get; set; }
}

And I would like to get most recent log per device. So I have this

from log in allLogs
                   group log by log.DeviceId
                   into l
                   select new {DeviceId = l.Key, TimeStamp = l.Max(s => s.TimeStamp)};

This works great except that it only includes the deviceId and the TimeStamp. I would also like to include the Lat and Lng fields from the Log as well.

But note these fields are not part of the key or the aggregate function. I.E I don’t want to group by these fields I just want then to be in the result set. So that I have the lat Lng of the most recent log per device.

Answer 1

This would work in LINQ to Objects – I don’t know about LINQ to SQL etc:

var query = from log in allLogs
            group log by log.DeviceId into logs
            select logs.OrderByDescending(x => x.Timestamp).First();

In other words, sort each group, and take the latest.

Alternatively, definitely only in LINQ to Objects, you could use MaxBy from MoreLINQ:

var query = from log in allLogs
            group log by log.DeviceId into logs
            select logs.MaxBy(x => x.Timestamp);