using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        var items = new List<DummyData>()

        {

            new DummyData() { Id = 1, Parent = 0 },

            new DummyData() { Id = 2, Parent = 0 },

            new DummyData() { Id = 3, Parent = 0 },

            new DummyData() { Id = 4, Parent = 0 },

            new DummyData() { Id = 5, Parent = 0 },

            new DummyData() { Id = 6, Parent = 0 },

            new DummyData() { Id = 7, Parent = 0 },

            new DummyData() { Id = 11, Parent = 1 },

            new DummyData() { Id = 111, Parent = 11 },

            new DummyData() { Id = 1111, Parent = 111 },

            new DummyData() { Id = 22, Parent = 2 },

            new DummyData() { Id = 222, Parent = 22 },

            new DummyData() { Id = 12, Parent = 1 },

            new DummyData() { Id = 121, Parent = 12},

            new DummyData() { Id = 1221, Parent = 121 },

            new DummyData() { Id = 13, Parent = 1 },

            new DummyData() { Id = 131, Parent = 13 },

            new DummyData() { Id = 1311, Parent = 131 },

            new DummyData() { Id = 41, Parent = 4 },

            new DummyData() { Id = 441, Parent = 41 },

            new DummyData() { Id = 4441, Parent = 441 },

            new DummyData() { Id = 42, Parent = 2 },

            new DummyData() { Id = 442, Parent = 42 },

            new DummyData() { Id = 4442, Parent = 442 },

        };

        var mapped = items.Where(i => i.Parent == 0).Select(i => new Dummy() {

            Id = i.Id,

            Dummies = items.Where(i_2 => i_2.Parent == i.Id).Select(i_2 => 

                new Dummy() {

                    Id = i_2.Id,

                    Dummies = items.Where(i_3 => i_3.Parent == i_2.Id).Select(i_3 => 

                        new Dummy() {

                            Id = i_3.Id,

                            Dummies = items.Where(i_4 => i_4.Parent == i_3.Id).Select(i_4 => 

                                new Dummy() 

                                { 

                                    Id = i_4.Id,

                                    Dummies = new List<Dummy>()

                                }).ToList()

                        }).ToList()

                }).ToList()

        }).ToList();

        var recursiveMapped = BuildTree(items);

        Console.WriteLine("Brute force mapping:");

        mapped.ForEach(d => Console.Write(d.ToString() + "\n\n\n"));

        Console.WriteLine("Recursive mapping:");

        recursiveMapped.ForEach(d => Console.Write(d.ToString() + "\n\n\n"));

    }

    public static List<Dummy> BuildTree(List<DummyData> source, int parent = 0)

    {

        return source.Where(i => i.Parent == parent).Select(i => new Dummy() {

            Id = i.Id,

            Dummies = BuildTree(source, i.Id)

        }).ToList();

    }

}

​

public class DummyData

{

    public int Id { get; set; } 

    public int Parent { get; set; }

}

public class Dummy

{

    public int Id { get; set; } 

    public List<Dummy> Dummies { get; set; }

    public string ToString(string indent = "\t") {

        var output = indent + this.Id.ToString();

        Dummies.ForEach(d => {

            output += ": \n" + d.ToString();

        });

        return output;

    }

}

​