Question

Oh, I just find that the error is caused by another part of code.
Case closed.

I have 2 tables

1- userinfo

id uid name 
1 11 Billy
2 22 Paul
3 33 Joshua

2- Score

 id uid score
 1  11  30
 2  22  40
 3  11  50
 4  11  60
 5  33  20
 6  33  70
 7  33  80

I have a class called ScoreUser

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

I want to use linq to query the above two tables, get the maximum score of each user and map it into the ScoreUser Object.
I use the following code:

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

However, this code does does not work. It produces 7 objects instead of 3.
What should I do?

Answer 1

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