Merge Two Lists of Lists

Question

I would like to merge two Lists of Lists of a certain class that share the same key

Let's say that I have the class :

public class Album {

public string Name { get ; set; }
    public string Genre{ get ; set; }

}

and two Lists of Lists :

public List<List<Album>>  AlbumList1 ;
public List<List<Album>>  AlbumList1 ;

I would like to merge the lists in AlbumList1 and AlbumList2 that have the same Key .

For example if a List is called "Genre1" and another is called "Genre1" i would like to merge those two lists to create a unique list .

How can I perform this ?

http://stackoverflow.com/questions/13505672/simplest-way-to-form-a-union-of-two-lists would help — Miller, Feb 21 '14 at 18:08
Lists have indexes not keys... You'll have to explain how you want to match them in order for me to write useful code. — evanmcdonnal, Feb 21 '14 at 18:11
I'm confused - do you want a single list of `Albums` or a single list of `List`s? — D Stanley, Feb 21 '14 at 18:11
key is neither of this , it is the name of the list for exmple if I call a list "example" the key will be example List Example ; — user2505650, Feb 21 '14 at 18:12

score 3 · Accepted Answer · answered Feb 21 '14 at 18:12

It would probably look a bit like this:

var results = albumList1
    .SelectMany(l => l)
    .Concat(albumList2.SelectMany(l => l)
    .GroupBy(l => l.Name, g => g.ToList())
    .ToList();

Or perhaps like this:

var results = albumList1
    .Join(albumList2, 
          l => l[0].Name,
          l => l[0].Name, 
          (l1, l2) => l1.Concat(l2).ToList())
    .ToList();

However, I'd also recommend you consider refactoring the code to use a IDictionary<string, IEnumerable<Album>> or an ILookup<string, Album> instead.

score 0 · Answer 2 · answered Feb 21 '14 at 18:42

Seems like you might want to refactor that outer list a bit out to a class. Something like this:

public class Genre
{
    public string Name { get; set; }
    public List<Album> Albums { get; set; }
}

public class Album
{
    public string Genre { get; set; }
    public string Name { get; set; }
}

After that, you can then create a comparer (simplified)

public class GenreComarer : IEqualityComparer<Genre>
{
    public bool Equals(Genre x, Genre y)
    {
        return x.Name.Equals(y.Name);
    }

    public int GetHashCode(Genre obj)
    {
        return obj.Name.GetHashCode();
    }
}

public class AlbumComarer : IEqualityComparer<Album>
{
    public bool Equals(Album x, Album y)
    {
        return x.Name.Equals(y.Name);
    }

    public int GetHashCode(Album obj)
    {
        return obj.Name.GetHashCode();
    }
}

Then, it's a simple join - adding back in the missing Genres.

    List<Genre> unified = list1.Join(list2, 
        e => e.Name, 
        e => e.Name, 
        (genre1, genre2) => new Genre 
            { 
                Name = genre1.Name, 
                Albums = genre1.Albums.Union(genre2.Albums, new AlbumComarer()).ToList() 
            }
        ).ToList();
    unified.AddRange(list2.Except(list1, new GenreComarer()));
    unified.AddRange(list1.Except(list2, new GenreComarer()));

Merge Two Lists of Lists

2 Answers2