LINQ query for retrieving data from list

Question

I have List collection of Message objects.

public class Message
{
    public int Id { get; set; }
    public string Body { get; set; }
    public string Sender { get; set; }
    public DateTime Timestamp { get; set; }
}

I want to get only one message with most recent Timestamp for each sender. How do I do it using LINQ?

Answer 1

You need to group by Sender and then get the Max Timestamp from each group like:

var query = list.GroupBy(r => r.Sender)
                .Select(grp => new
                {
                    Sender = grp.Key,
                    RecentTimeStamp = grp.Max(r => r.Timestamp)
                });

Or you can sort the TimeStamp in group by descending order and get the first element like:

var query = list.GroupBy(r => r.Sender)
                .Select(grp => new
                {
                    Sender = grp.Key,
                    RecentTimeStamp = grp.OrderByDescending(r => r.Timestamp).FirstOrDefault()
                });

Answer 2

var q = from n in table
        group n by n.Senderinto g
        select g.OrderByDescending(t=>t.Timestamp).FirstOrDefault();