gistfile1.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Issue
    {
        public string ID { get; set; }
        public string Date { get; set; }
        public string ParentID { get; set; }

        public override string ToString()
        {
            return "id=" + ID + " date=" + Date + " parent=" + ParentID;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var dataList = new List<Issue>()
            {
                new Issue(){ID = "0", Date = "0", ParentID = ""},
                new Issue(){ID = "1", Date = "3", ParentID = ""},
                new Issue(){ID = "2", Date = "4", ParentID = ""},
                    new Issue(){ID = "20", Date = "6", ParentID = "2"},
                    new Issue(){ID = "21", Date = "2", ParentID = "2"},
                    new Issue(){ID = "22", Date = "3", ParentID = "2"},
                        new Issue(){ID = "220", Date = "4", ParentID = "22"},
                        new Issue(){ID = "221", Date = "5", ParentID = "22"},
                new Issue(){ID = "3", Date = "5", ParentID = ""},
                new Issue(){ID = "4", Date = "8", ParentID = ""},
                new Issue(){ID = "5", Date = "2", ParentID = ""},
                    new Issue(){ID = "50", Date = "3", ParentID = "5"},
                new Issue(){ID = "6", Date = "1", ParentID = ""},
                    new Issue(){ID = "60", Date = "2", ParentID = "6"},
                new Issue(){ID = "7", Date = "9", ParentID = ""},
            };

            var sortedList = test2(dataList);

            sortedList.ForEach(i => Console.WriteLine(i));
            Console.Read();
        }

        static private List<Issue> test2(List<Issue> list)
        {
            // リスト全体を日付でソートする
            // 日付順を保ったまま親がいないissueの新しいリストを作る
            // 残ったissueは親を探して日付順を保ったまま親の次の位置に挿入する

            // IDからissueを検索するためのテーブル
            var table = list.ToDictionary(issue => issue.ID, issue => issue);

            // 日付でソートする
            var sortedList = list.OrderBy(issue => issue.Date).ToList();

            // 新しいリストを作成
            var parentChildRelationship = new List<Issue>();

            // 親のいないissueを探して新しいリストに追加
            var parentsList = sortedList.Where(issue => !table.ContainsKey(issue.ParentID));
            parentChildRelationship.AddRange(parentsList);

            // 追加したissueはソート済みリストから削除する
            sortedList.RemoveAll(issue => parentsList.Contains(issue));

            // ソート済みリストの要素がなくなるまで繰り返す
            while (sortedList.Count != 0)
            {
                // リストの先頭の親の位置を探す
                var parentID = sortedList[0].ParentID;
                var parentIndex = parentChildRelationship.IndexOf(table[parentID]);

                // 処理対象の親を持つissueを探す
                var childList = sortedList.Where(issue => issue.ParentID == parentID);

                // 子のリストを親の次の位置にインサートする
                parentChildRelationship.InsertRange(parentIndex+1, childList);

                // ソート済みリストからインサート済みの子を削除する
                sortedList.RemoveAll(issue => childList.Contains(issue));
            }

            return parentChildRelationship;
        }
   }
}