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シリアライズ：バージョンによって内容が変わったらどうする？

#define v1
#define v2
#define w
#define r
using System;
using System.Collections.Generic;
using System.IO;
using System.Runtime.Serialization;
using System.Runtime.Serialization.Formatters.Binary;
namespace serialize {
  class Program {
    static void Main(string[] args) {
      SortedDictionary<ulong, status> d;
      var f = new BinaryFormatter();
#if (v1 && w)
      d = new SortedDictionary<ulong, status>();
      d.Add(1, new status() { id = 1, text = "1" });
      d.Add(2, new status() { id = 2, text = "2" });
      d.Add(3, new status() { id = 3, text = "3" });
      using (var s = new FileStream("test.dat", FileMode.Create)) {
        f.Serialize(s, d);
      }
#elif (v2 && w)
      d = new SortedDictionary<ulong, status>();
      d.Add(1, new status() { id = 11, text = "aa", created_at = new DateTime(1, 2, 3) });
      d.Add(2, new status() { id = 12, text = "bb", created_at = new DateTime(2010, 1, 1, 12, 34, 56) });
      d.Add(3, new status() { id = 13, text = "cc", created_at = DateTime.Now });
      using (var s = new FileStream("test.dat", FileMode.Create)) {
        f.Serialize(s, d);
      }
#elif r
      using (var s = new FileStream("test.dat", FileMode.Open)) {
        d = (SortedDictionary<ulong, status>)f.Deserialize(s);
      }
      foreach (var v in d) {
        Console.WriteLine(v);
      }
#endif
    }
  }
  [Serializable]
  public class status {
    public ulong id;
    public string text;
#if v1
    public override string ToString() {
      return string.Format("id:{0} text:{1}", id, text);
    }
#elif v2
    [OptionalField(VersionAdded = 2)]
    public DateTime created_at;
    [OnDeserializing] // 初期値を入れる
    internal void OnDeserializingMethod(StreamingContext context) {
      created_at = DateTime.Now;
    }
    [OnDeserialized] // 読み込んだ他のデータを参照して値を入れるならこっち
    internal void OnDeserializedMethod(StreamingContext context) {
      if (id == 1)
        created_at = DateTime.Now.Subtract(new TimeSpan(1, 0, 0, 0));
    }
    public override string ToString() {
      return string.Format("id:{0} text:{1} created_at:{2}", id, text, created_at);
    }
#endif
  }
}

こんな感じ
VersionAddedの値はどこで使うんだろう？