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, int maxDepth)

    {

        var results = Enumerable.Range(0, maxDepth+1 ).Select( i => new List<FlatData>() ).ToList();

        results[0].Add

        (

            This.Single( x => x.Id == rootId ) 

        );

        for (int level = 1; level <= maxDepth; level++)

        {

            results[level].AddRange

            (

                results[level-1].SelectMany

                ( 

                    x => This.Where( y => y.ParentId == x.Id )

                )

            );

        }

        return results.SelectMany( x => x );

    }

}

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, 2);

        foreach (var f in filtered)

        {

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

        }

    }

}