How to use LINQ to select into an object?

Question

I have data that looks like so:

UserId   |  SongId
--------   --------
1          1
1          4
1          12
2          95

I also have the following class:

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

What I would like to do is use LINQ to select from my data to create a collection of SongsForUser objects. Below is what I have come up with so far:

var userCombos = songs.UserSongs.Select(x => new SongsForUser() { User = x.UserId, 
                                                                  Songs = /*What goes here?*/ });

How would I go about populating my Songs List?

So the result should be two SongsForUser objects. For user 1 it would have 3 items in the Songs list. For user 2 it would have 1 item in the Songs list.

ForEveR · Accepted Answer · 2012-07-19T05:36:56.460

46

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

edited Jul 19 '12 at 05:36

answered Jul 19 '12 at 05:28

ForEveR

55,233
2
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2

Why select the song ID only within the projection, rather than as part of the grouping? – Jon Skeet Jul 19 '12 at 05:34
2

This is throwing an error on the `ToList` portion. I believe it may be because this is Linq to Entities. Error: `LINQ to Entities does not recognize the method 'System.Collections.Generic.List`1[System.String] ToList[String](System.Collections.Generic.IEnumerable`1[System.String])' method, and this method cannot be translated into a store expression.` – Abe Miessler Jul 19 '12 at 05:34
@AbeMiessler make sure you have `using System.Linq` up top. – Eren Ersönmez Jul 19 '12 at 05:37
Nice. Came up with a similar solution. +1 – Mare Infinitus Jul 19 '12 at 05:42
@AbeMiessler: It would have been nice if you'd mentioned LINQ to Entities to start with... – Jon Skeet Jul 19 '12 at 05:47
Yes, I realize that was a mistake. I apologize. – Abe Miessler Jul 19 '12 at 05:48
@AbeMiessler: If the `ToList` is failing for you, see the edit to my answer for a potential workaround. – Jon Skeet Jul 19 '12 at 05:49

Jon Skeet · Answer 2 · 2012-07-19T05:49:08.697

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.

score 1 · Answer 3 · answered Jul 19 '12 at 05:35

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.

score 0 · Answer 4 · answered Feb 05 '19 at 00:09

0

In its simplest form you can just:

List<MapPoint> points = db.PropertyResearches.Where(a => a.deptId == 66).Select(x => new MapPoint { property = x.notes.Substring(0, 10), latitude = x.lat, longitude = x.@long }).ToList();

answered Feb 05 '19 at 00:09

pat capozzi

1,412
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How to use LINQ to select into an object?

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