How To Project a Line Number Into Linq Query Results

问题

How can I project the row number onto the linq query result set.

Instead of say:

field1, field2, field3

I would like:

1, field1, field2, field3

2, field1, field2, field3

Here is my attempt at this:

public List<ScoreWithRank> GetHighScoresWithRank(string gameId, int count)
{
    Guid guid = new Guid(gameId);
    using (PPGEntities entities = new PPGEntities())
    {
        int i = 1;
        var query = from s in entities.Scores
                    where s.Game.Id == guid
                    orderby s.PlayerScore descending
                    select new ScoreWithRank()
                    {
                        Rank=i++,
                        PlayerName = s.PlayerName,
                        PlayerScore = s.PlayerScore
                    };
        return query.ToList<ScoreWithRank>();
    }
}

Unfortunately, the \"Rank=i++\" line throws the following compile-time exception:

\"An expression tree may not contain an assignment operator\"

回答1:

Well, the easiest way would be to do it at the client side rather than the database side, and use the overload of Select which provides an index as well:

public List<ScoreWithRank> GetHighScoresWithRank(string gameId, int count)
{
    Guid guid = new Guid(gameId);
    using (PPGEntities entities = new PPGEntities())
    {
        var query = from s in entities.Scores
                    where s.Game.Id == guid
                    orderby s.PlayerScore descending
                    select new
                    {
                        PlayerName = s.PlayerName,
                        PlayerScore = s.PlayerScore
                    };

        return query.AsEnumerable() // Client-side from here on
                    .Select((player, index) => new ScoreWithRank()
                            {
                                PlayerName = player.PlayerName,
                                PlayerScore = player.PlayerScore,
                                Rank = index + 1;
                            })
                    .ToList();

    }
}

回答2:

Ok, that did the trick. Thanks.

Here is my final code...

Server:

public List<Score> GetHighScores(string gameId, int count)
{
    Guid guid = new Guid(gameId);
    using (PPGEntities entities = new PPGEntities())
    {
        var query = from s in entities.Scores
                    where s.Game.Id == guid
                    orderby s.PlayerScore descending
                    select s;
        return query.ToList<Score>();
    }                                                                      
}

Client:

void hsc_LoadHighScoreCompleted(object sender, GetHighScoreCompletedEventArgs e)
{
    ObservableCollection<Score> list = e.Result;

    _listBox.ItemsSource = list.Select((player, index) => new ScoreWithRank()
                            {
                                PlayerName = player.PlayerName,
                                PlayerScore = player.PlayerScore,
                                Rank = index+=1
                            }).ToList();
}

回答3:

You could also make just a slight adjustment to your original code to get it working. Word of caution, if you databind or access the object again, the Rank will increment each time. In those cases the top answer is better.

let Rank = i++

and

Rank.ToString()

Full code:

public List<ScoreWithRank> GetHighScoresWithRank(string gameId, int count)
{
Guid guid = new Guid(gameId);
using (PPGEntities entities = new PPGEntities())
{
    int i = 1;
    var query = from s in entities.Scores
                let Rank = i++
                where s.Game.Id == guid
                orderby s.PlayerScore descending
                select new ScoreWithRank()
                {
                    Rank.ToString(),
                    PlayerName = s.PlayerName,
                    PlayerScore = s.PlayerScore
                };
    return query.ToList<ScoreWithRank>();
}

}

回答4:

This solution worked for me. http://www.dotnetfunda.com/articles/article1995-rownumber-simulation-in-linq.aspx

.Select((x, index) => new
{
     SequentialNumber = index + 1
    ,FieldFoo = x.FieldFoo                        
}).ToList();

回答5:

List<Emp> Lstemp = GetEmpList(); 
int Srno = 0; 
var columns = from t in Lstemp 
              orderby t.Name 
              select new { 
                  Row_number=++Srno, 
                  EmpID = t.ID, 
                  Name = t.Name, 
                  City = t.City 
              };

来源：https://stackoverflow.com/questions/365086/how-to-project-a-line-number-into-linq-query-results

标签

linq

row-number