Get the first record of a group in LINQ? [duplicate]

Answer 1

data
    .GroupBy(
        x => x.CardId, 
        (x, y) => new { 
            Key = x, 
            Value = y.OrderByDescending(z => z.DateTimeStamp).FirstOrDefault() 
        }
    );

This will group all the elements by CardId, then order the elements of each group by DateTimeStamp (descending), then pare down each group to only contain the first element. Finally, it returns an enumerable of "groups" (with the scare quotes since they're actually an anonymous type instead of an IGrouping) where each group has the one item you're seeking.

Answer 2

I find the query expression syntax easier to understand myself:

from x in data 
group x by x.CarId into grp
select new {
    CarId = x.CarId,
    Record = (from x in grp orderby z.DateTimeStamp descending).FirstOrDefault()
};