Linq – How To Project a Line Number Into Linq Query Results

linqrow-number

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

Instead of say:

field1, field2, field3

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"

Best Answer

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();

    }
}
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