using System;

using System.Linq;

using System.Reflection;

using System.Collections.Generic;

using System.Collections;

​

public class FlatData

{

    public int Id { get; set; }

    public int ParentId { get; set; }

    public string Text { get; set; }

}

​

    public static class ExtensionMethods

    {

        public static IEnumerable<FlatData> GetDescendents(this IEnumerable<FlatData> This, int rootId)

        {

            var rootItem = This.Single( x => x.Id == rootId );

            var queue = new Queue<FlatData>( new [] { rootItem } );

            while (queue.Count > 0)

            {

                var item = queue.Dequeue();

                yield return item;

                foreach (var child in This.Where( x => x.ParentId == item.Id )) 

                {

                    queue.Enqueue(child);

                }

            }

        }

    }

public class Program

{

    public static void Main()

    {

        FlatData[] elements = new FlatData[]

        {

           new FlatData {Id = 3, ParentId = 1, Text = "A"}, 

           new FlatData {Id = 4, ParentId = 1, Text = "D"},

           new FlatData {Id = 5, ParentId = 2, Text = "E"},

           new FlatData {Id = 7, ParentId = 2, Text = "G"},

           new FlatData {Id = 8, ParentId = 4, Text = "H"},

           new FlatData {Id = 9, ParentId = 8, Text = "H"},

           new FlatData {Id = 10, ParentId = 8, Text = "I"},

           new FlatData {Id = 11, Text = "I"},

        };

        var filtered = elements.GetDescendents(4).ToList();

        foreach (var f in filtered)

        {

            Console.WriteLine("{0} {1} {2}", f.Id, f.ParentId, f.Text);

        }

    }

}