How to write recursive lambda expression (LINQ query)

Question

I have an object:

public class Folder : DBObjectBase
{
    public string Name { get; set; }
    public List<FileEntry> Files { get; set; }
    public Folder ParentFolder { get; set; }
    public List<Folder> ChildFolders { get; set; }
}

and I've written a query that retrieves a folder structure and all files within each folder:

var results = DbContext.Set<Folder>()
            .Include(f => f.ParentFolder)
            .Include(f => f.ChildFolders)
            .Include(f => f.Files)
            .Include(f => f.ChildFolders.Select(f1 => f1.Files))
            .Include(f => f.ChildFolders.Select(f1 => f1.ChildFolders).Select(f2 => f.Files))
            .Include(f => f.ChildFolders.Select(f1 => f1.ChildFolders).Select(f2 => f.ChildFolders.Select(f3 => f3.Files)))
            .Include(f => f.ChildFolders.Select(f1 => f1.ChildFolders).Select(f2 => f.ChildFolders.Select(f3 => f3.ChildFolders.Select(f4 => f4.Files))))
            .Include(f => f.ChildFolders.Select(f1 => f1.ChildFolders).Select(f2 => f.ChildFolders.Select(f3 => f3.ChildFolders.Select(f4 => f4.ChildFolders.Select(f5 => f5.Files)))))
            .Include(f => f.ChildFolders.Select(f1 => f1.ChildFolders).Select(f2 => f.ChildFolders.Select(f3 => f3.ChildFolders.Select(f4 => f4.ChildFolders.Select(f5 => f5.ChildFolders.Select(f6 => f6.Files))))))
            .Where(f => f.ParentFolder == null);

The above returns exactly what I need, but I don't like the code, cause it causes problems if I would like to add multiple more layers to the folder structure.

Any ideas how I could write this, so I get all Child folders, and all files despite the number of layers I have in my folder structure?

Have a look at this: http://stackoverflow.com/questions/61143/recursive-lambda-expression-to-traverse-a-tree-in-c-sharp — Max Yankov, May 06 '14 at 08:58
Can you clarify: Are you using a DB and you want this to be translated to one DB command? — Christoph Fink, May 06 '14 at 09:02
I am using a db and this is the basic query I'm after SELECT * FROM Folders f LEFT JOIN FileEntries fe On f.Id = fe.Folder_Id — h3li0s, May 06 '14 at 09:14

Christoph Fink · Accepted Answer · 2014-05-06T09:01:16.920

Sadly there is no recursive support in LINQ AFAIK (I faced the same problem), but you at least can shorten it to:

var results = DbContext.Set<Folder>()
            .Include(f => f.ParentFolder)
            .Include(f => f.Files)
            .Include(f => f.ChildFolders.Select(f1 => f1.ChildFolders).Select(f2 => f.ChildFolders.Select(f3 => f3.ChildFolders.Select(f4 => f4.ChildFolders.Select(f5 => f5.ChildFolders.Select(f6 => f6.Files))))))
            .Where(f.ParentFolder == null);

All "lower levels" are also included if you select the "higher level".

I THINK this is because SQL also has no support for this...

If you are fine with executing this "in code" using multiple queries to the DB (or you not using a DB at all) see golergka's comment.

score 0 · Answer 2 · answered May 06 '14 at 08:58

You have a tree of folders, and you'd like to look for all files recursively. This can be done using algorithms like depth-first search:

public static IEnumerable<FileEntry> GetAllFiles(Folder folder)
{
    foreach(var file in folder.Files)
    {
        yield return file;
    }
    foreach(var nestedFolder in folder.ChildFolders)
    {
        foreach(var file in GetAllFiles(nestedFolder))
        {
            yield return file;
        }
    }
}

This isn't very pretty in C# since it has no support for yielding multiple results, but it works. Also, if you have huge quantities of files, or very nested folders, you might want to implement this algorithm using more efficient techniques (e.g. with a Queue).

Suspecting by `DBObjectBase` he is using a DB backend so he wants to do this not via code... — Christoph Fink, May 06 '14 at 08:59

How to write recursive lambda expression (LINQ query)

2 Answers2