public class TreeTableItem
    {
        public int Id { get; set; }
        public int ParentId { get; set; }
        public string Description { get; set; }
        public override string ToString()
        {
            var template = "Id-{0};ParentId-{1};Description-{2};";
            return string.Format(template, Id, ParentId, Description);
        }
    }

 public class TreeHelper
    {
        public static IEnumerable<Tuple<int, int, int, string>> GetChildren(IEnumerable<TreeTableItem> tab, int id, int level)
        {
            foreach (var e in tab.Where(x => x.Id == id))
            {
                yield return Tuple.Create(e.Id, e.ParentId, level, e.Description);                
            }
            foreach (var e in tab.Where(x => x.ParentId == id))
            {
                foreach (var u in GetChildren(tab, e.Id, level + 1))
                {
                    yield return u;
                }
            }
        }
    }

 var data = new TreeTableItem[] 
            {
                new TreeTableItem(){Id=1,ParentId=-1,Description="Description1"},
                new TreeTableItem(){Id=2,ParentId=1,Description="Description2"},
                new TreeTableItem(){Id=3,ParentId=1,Description="Description3"},
                new TreeTableItem(){Id=4,ParentId=3,Description="Description4"},
                new TreeTableItem(){Id=5,ParentId=2,Description="Description5"},
                new TreeTableItem(){Id=6,ParentId=2,Description="Description6"}
            };
            var res = TreeHelper.GetChildren(data, 2, 1);

            foreach (var d in res)
            {
                Console.WriteLine(d + "level-" + d.Item3);
            }