using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public class Data

    {

        public int? Col1

        {

            get;

            set;

        }

​

        public int? Col2

        {

            get;

            set;

        }

​

        public Data(int? col1, int? col2)

        {

            Col1 = col1;

            Col2 = col2;

        }

    }

​

    public static IEnumerable<int> GetRecusive(List<Data> source, int val)

    {

        var q = new Queue<int>();

        q.Enqueue(val);

        while (q.Any())

        {

            var current = q.Dequeue();

            var potential = source.Where(x => x.Col1 == current && x.Col2 != null);

            foreach (var item in potential)

            {

                yield return item.Col2.Value;

                q.Enqueue(item.Col2.Value);

            }

        }

    }

​

    public static void Main()

    {

        // some ill defined test data

        var list = new List<Data>()

        {new Data(1, 14), new Data(2, 17), new Data(1, 2), new Data(4, null), new Data(1, 3), new Data(20, 4), new Data(17, null), new Data(2, 20), new Data(14, 6), new Data(14, null), new Data(6, null), new Data(3, 30), new Data(3, 19), };

        var results = GetRecusive(list, 1);

        // compose as a comma separated list 

        Console.WriteLine(string.Join(", ", results));

    }

}