using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public class Comp{ public string Id{get;set;} public string ParentId{get;set;} }

    public static List<Comp> data=new List<Comp>(){

            new Comp(){Id = "1", ParentId = ""},

            new Comp(){Id = "2", ParentId = "1"},

            new Comp(){Id = "3", ParentId = "1"},

            new Comp(){Id = "4", ParentId = "2"},

            new Comp(){Id = "5", ParentId = "2"},

            new Comp(){Id = "6", ParentId = "3"},

            new Comp(){Id = "7", ParentId = "10"},

            new Comp(){Id = "8", ParentId = "11"},

            new Comp(){Id = "9", ParentId = ""},

            new Comp(){Id ="10", ParentId = ""},

            new Comp(){Id = "11", ParentId = ""}

        };

​

    public static List<string> GetChildsFor(string Id)

    {

        List<string> result=new List<string>();

        // Direct childs for this id

        var childs=data.Where(o=>o.ParentId==Id).Select(o=>o.Id);

        foreach(var childId in childs)

        {

            // Add them to the list

            result.Add(childId);

            // And start again using each child as a root

            result.AddRange(GetChildsFor(childId));

        }

        return result;

    }

    public static void Main()

    {

        foreach(var testId in new string[]{"1","2","6"})

        {

            var result=GetChildsFor(testId);

            Console.WriteLine(testId + ": " + string.Join(", ", result.OrderBy(o=>o)));

        }

    }

}