using System;

using System.Collections.Generic;

using System.Linq;

​

public class Hierarchy

{

    public Hierarchy(string iD, string name, int level, string parentID, string topParent)

    {

        ID = iD;

        Name = name;

        Level = level;

        ParentID = parentID;

        Children = new HashSet<Hierarchy>();

    }

    public string ID { get; set; }

    public string Name{ get; set; }

    public int Level { get; set; }

    public string ParentID { get; set; }

    public ICollection<Hierarchy> Children { get; set; }

}

​

public class Program

{

    static Hierarchy CreateTree(IEnumerable<Hierarchy> Nodes)

    {

        var idToNode = Nodes.ToDictionary(n => n.ID, n => n);

​

        Hierarchy root = null;

        foreach (var n in Nodes)

        {

            if (n.ParentID == null)

            {

                if (root != null)

                {

                    //there are multiple roots in the data

                }

                root = n;

                continue;

            }

​

            Hierarchy parent = null;

            if (!idToNode.TryGetValue(n.ParentID, out parent))

            {

                //Parent doesn't exist, orphaned entry

            }

​

            parent.Children.Add(n);

            Console.WriteLine("ReferenceEquals: {0}", Object.ReferenceEquals(parent, root));

        }

​

        if (root == null)

        {

            //There was no root element

        }

        return root;

    }

    public static void Main()

    {

        Console.WriteLine("Test #1");

        List<Hierarchy> l = new List<Hierarchy>();

        l.Add(new Hierarchy("295152","name1",1,null, null)); // <-root node at the top of the list

        l.Add(new Hierarchy("12345","child1",2,"295152", null));

        l.Add(new Hierarchy("54321","child2",2,"295152", null));

        l.Add(new Hierarchy("44444","child1a",3,"12345", null));

        l.Add(new Hierarchy("33333","child1b",3,"12345", null));

        l.Add(new Hierarchy("22222","child2a",3,"54321", null));

        l.Add(new Hierarchy("22221","child2b",3,"54321", null));

        l.Add(new Hierarchy("22002","child2c",3,"54321", null));

        l.Add(new Hierarchy("20001","child2a2",4,"22222", null));

        l.Add(new Hierarchy("20101","child2b2",4,"22222", null));       

        CreateTree(l);

        Console.WriteLine("\nTest #2");

        l = new List<Hierarchy>();

        l.Add(new Hierarchy("12345","child1",2,"295152", null));

        l.Add(new Hierarchy("54321","child2",2,"295152", null));

        l.Add(new Hierarchy("44444","child1a",3,"12345", null));

        l.Add(new Hierarchy("33333","child1b",3,"12345", null));

        l.Add(new Hierarchy("22222","child2a",3,"54321", null));

        l.Add(new Hierarchy("22221","child2b",3,"54321", null));

        l.Add(new Hierarchy("22002","child2c",3,"54321", null));

        l.Add(new Hierarchy("20001","child2a2",4,"22222", null));

        l.Add(new Hierarchy("20101","child2b2",4,"22222", null));

        l.Add(new Hierarchy("295152","name1",1,null, null)); // <-root node at the bottom of the list

        CreateTree(l);

    }

}