Using Linq to select maximum value in a group

for someone's flavor, providing here two LINQ using lambda expressions to select maximum value in a group

LINQ using lambda expressions

qry = Scores.Join(UserInfos, s => s.uid, i => i.uid, (s, i) => new { s, i })
     .GroupBy(g => new { g.s.uid, g.i.name })
     .Select(g => new ScoreUser
     {
        uid = g.Key.uid,
        name = g.Key.name,
        score = g.Max(p => p.s.score)
     });

LINQ(Lambda) using LastOrDefault() to get Max() value, so the class ScoreUser() can be eliminated here.

var qry = Scores.Join(UserInfos, s => s.uid, i => i.uid, (s, i) => new { s, i })
         .GroupBy(g => new { g.s.uid, g.i.name })
         .Select(g => new { su = g.OrderBy(i => i.s.score) })
         .Select(x => x.su.LastOrDefault()).ToList();

Both get same results as LINQ with query syntax.

This document may be interested LINQ Query Syntax versus Method Syntax.

var list = records.GroupBy(p => p.Year, (key, g) => g.OrderByDescending(y => y.Month).First()).ToList();

Above query will return a list that includes the highest month item by grouping the years.

You are also grouping by score when it should be the aggregator. Try this:

from s in Scores
join i in UserInfos on s.uid equals i.uid
group by new { s.uid, i.name } into g
select new ScoreUser
{
   uid = g.Key.uid
   name = g.Key.name,
   score = g.Max(p => p.score)
}

(update)

I see you found the problem. However I leave you here a test to this query:

    class UserInfo
    {
        public int Id { get; set; }
        public int UId { get; set; }
        public string Name { get; set; }
    }

    class Score
    {
        public int Id { get; set; }
        public int UId { get; set; }
        public int SScore { get; set; }
    }

    public class ScoreUser
    {
        public int uid { get; set; }
        public string name { get; set; }
        public int score { get; set; }

        public override string ToString()
        {
            return string.Format("UId:{0} Name:{1} Score:{2}", uid, name, score);
        }
    }


    static void Main(string[] args)
    {

        List<UserInfo> infos = new List<UserInfo>()
        {
            new UserInfo {Id = 1, UId = 11, Name = "Billy"},
            new UserInfo {Id = 2, UId = 22, Name = "Paul"},
            new UserInfo {Id = 3, UId = 33, Name = "Joshua"}
        };

        List<Score> scores = new List<Score>()
        {
            new Score {Id = 1, UId = 11, SScore = 30},
            new Score {Id = 2, UId = 22, SScore = 40},
            new Score {Id = 3, UId = 11, SScore = 50},
            new Score {Id = 4, UId = 11, SScore = 60},
            new Score {Id = 5, UId = 33, SScore = 20},
            new Score {Id = 6, UId = 33, SScore = 70},
            new Score {Id = 7, UId = 33, SScore = 80}
        };

        var qry = from s in scores
                  join i in infos on s.UId equals i.UId
                  group s by new { s.UId, i.Name } into g
                  select new ScoreUser
                  {
                      uid = g.Key.UId,
                      name = g.Key.Name,
                      score = g.Max(p => p.SScore)
                  };

        foreach (var su in qry)
        {
            Console.WriteLine(su);
        }
    }

Prints:

UId:11 Name:Billy Score:60
UId:22 Name:Paul Score:40
UId:33 Name:Joshua Score:80