using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var lista = new List<Nodo> {

            new Nodo { Id = 1, ParentId = null, DisplayOrder = 1, Text = "Root" },

            new Nodo { Id = 2, ParentId = 1, DisplayOrder = 1, Text = "Cat 1" },

            new Nodo { Id = 3, ParentId = 1, DisplayOrder = 2, Text = "Cat 2" },

            new Nodo { Id = 8, ParentId = 1, DisplayOrder = 1, Text = "Cat 3" },

            new Nodo { Id = 4, ParentId = 2, DisplayOrder = 1, Text = "Subcat 1.1" },

            new Nodo { Id = 5, ParentId = 2, DisplayOrder = 2, Text = "Subcat 1.2" },

            new Nodo { Id = 6, ParentId = 5, DisplayOrder = 1, Text = "Subcat 1.2.1" },

            new Nodo { Id = 7, ParentId = 3, DisplayOrder = 1, Text = "Subcat 2.1" }

        };

        BuildTreeView(lista, 1, 0);

    }

​

    public class Nodo

    {

        public int Id;

        public int ? ParentId;

        public int DisplayOrder;

        public string Text;

    }

​

    public static void BuildTreeView(IEnumerable<Nodo> data, int ? parentid = 1, int level = 0, string accumulatore = "")

    {

        var nodes = data.Where(n => n.ParentId == parentid).OrderBy(n => n.DisplayOrder);

        var separatore = "";

        if (nodes.Any())

        {

            if (nodes.First().ParentId != null)

            {

                if (level > 0)

                {

                    separatore = " >> ";

                }

            }

​

            if (level == 0){

                Console.Write("<select>");

            }

            foreach (var node in nodes)

            {

                BuildTreeView(data, node.Id, level + 1, accumulatore + separatore + node.Text);

            }

            if (level == 0){

                Console.Write("\r\n</select>");

            }

        }

        else

        {

            Console.Write(string.Format("\r\n<option value=\"{0}\">{1}</option>", data.Single(nodo => nodo.Id == parentid.Value).Text, accumulatore));

        }

    }

}