How to use LINQ to select into an object?

Question

songs.UserSongs.GroupBy(x => x.User).Select(g => new SongsForUser() 
{ 
    User = g.Key,
    Songs = g.Select(s => s.SongId).ToList()
});

I suspect you want:

var songsByUser = songs.UserSongs
                       .GroupBy(song => song.UserId, song => song.SongId)
                       .Select(g => new SongsForUser { User = g.Key,
                                                       Songs = g.ToList() });

To explain, after the GroupBy you'll have a bunch of groups, where the key of each group is the user ID, and the values within the group are the song IDs:

Key = 1, Values = 1, 4, 12
Key = 2, Value = 95

Then you're just converting that into your SongsForUser type. Note that you don't need to explicitly include the () when calling the constructor in an object initializer - it's implicit unless you need to specify constructor arguments.

You could do this all in one GroupBy call, by the way:

var songsByUser = songs.UserSongs
         .GroupBy(song => song.UserId, song => song.SongId,
                  (user, ids) => new SongsForUser { User = user,
                                                    Songs = ids.ToList() });

Personally I usually find a separate Select call to be more readable.

You can also do all of this with a query expression:

var songsByUser = from song in songs.UserSongs
                  group song.SongId by song.UserId into g
                  select new SongsForUser { User = g.Key, Songs = g.ToList() };

EDIT: The above is "provider-neutral" but it sounds like it's not working with LINQ to Entities. You may be able to get it to work like this:

var songsByUser = songs.UserSongs
                       .GroupBy(song => song.UserId, song => song.SongId)
                       .AsEnumerable()
                       .Select(g => new SongsForUser { User = g.Key,
                                                       Songs = g.ToList() });

The AsEnumerable call will force the grouping to be done in the database, but the final projection (including the ToList call) to be done locally. You should check the generated SQL for efficiency though.

Lets say you have the following:

public class SongsForUser
{
    public int UserId;
    public List<int> Songs;
}

Then a function like this one here will do. The list is just there to have some data to test with.

    public void Group()
    {
        List<Tuple<int, int>> SongRelations = new List<Tuple<int, int>>();

        SongRelations.Add(new Tuple<int, int>(1, 1));
        SongRelations.Add(new Tuple<int, int>(1, 4));
        SongRelations.Add(new Tuple<int, int>(1, 12));
        SongRelations.Add(new Tuple<int, int>(2, 95));

        var list = SongRelations.GroupBy(s => s.Item1)
                                .Select(r => new SongsForUser()
                                {
                                    UserId = r.Key,
                                    Songs = r.Select(t => t.Item2).ToList(),
                                });
    }

list contains 2 items of type SongsForUser afterwards. One with user 1 and a list of songs containing 1, 4 and 12 and one with user 2 and a list of songs containing 95.