How to do reference fixup during deserialization in C#?

Question

How can one do reference fixup (post-processing) using the C# serialization framework?

I have an object graph with objects referencing other objects. They all implement the ISerializable interface and they all have instance ID's, so representing the references in the serialized state is easy.

The krux is that when the deserialization constructor is called, all objects being referenced by that object might not have been deserialized, so the references can't be set to valid objects. And I can't find any way to hook into a post-processing step in the C# serialization framework to do the reference fixup. Is there a way to do it?

---

As per request, here is a contrived class that I think highlights the problem.

[Serializable]
public class Pony : ISerializable
{
  public int Id { get; set; }
  public string Name { get; set; }
  public Pony BFF { get; set; }

  public Pony() {}
  private Pony(SerializationInfo info, StreamingContext context) {
    Id = info.GetInt32("id");
    Name = info.GetString("name");
    var bffId = info.GetInt32("BFFId");
    BFF = null; // <===== Fixup! Find Pony instance with id == bffId
  }

  public void GetObjectData(SerializationInfo info, StreamingContext ontext) {
    info.AddValue("id", Id);
    info.AddValue("name", Name);
    info.AddValue("BFFId", BFF.Id);
  }
}

And here is the (de)serialization code:

var rd = new Pony { Id = 1, Name = "Rainbow Dash" };
var fs = new Pony { Id = 2, Name = "Fluttershy", BFF = rd };
rd.BFF = fs;
var ponies = new List<Pony>{ rd, fs };
Stream stream = new MemoryStream();
var formatter = new BinaryFormatter();
formatter.Serialize(stream, ponies);
stream.Seek(0, SeekOrigin.Begin);
var deserializedPonies = (List<Pony>)formatter.Deserialize(stream);

---

This question doesn't solve my problem: .net XML Serialization - Storing Reference instead of Object Copy

I would like to use the BinaryFormatter + ISerializable framework for the serialization and not switch to XmlFormater.

Answer 1

There is an Attribute for this purpose.

Implement the following method in any object you want to deserialize:

[OnDeserialized]
internal void OnDeserializedMethod(StreamingContext context) {

}

There are some more Attributes in System.Runtime.Serialization which might help you.

EDIT

I have modified your code a bit:

[Serializable]
  public class Pony  {
    public int Id {
      get; set;
    }
    public string Name {
      get; set;
    }
    public Pony BFF {
      get; set;
    }

    public Pony() {
    }

    [OnDeserialized]
    internal void OnDeserializedMethod(StreamingContext context) {
      Console.WriteLine(this.Id + " " + this.Name + " " + this.BFF?.Name);
    }
  }

TestMethod:

var rd = new Pony { Id = 1, Name = "Rainbow Dash" };
      var fs = new Pony { Id = 2, Name = "Fluttershy", BFF = rd };
      rd.BFF = fs;
      var ponies = new List<Pony> { rd, fs };

      object returnValue;
      using (var memoryStream = new MemoryStream()) {
        var binaryFormatter = new BinaryFormatter();
        binaryFormatter.Serialize(memoryStream, ponies);
        memoryStream.Position = 0;
        returnValue = binaryFormatter.Deserialize(memoryStream);
      }
      var xx = (List<Pony>)returnValue;

As you can see, i removed the ISerializable Interface, the private contructor and your GetObjectData - Method.

I did that, because i dont think you really need it as you havent stated, that you ahd made your implementation of (De)Serialization.

This post is another source of information

EDIT 2 (Testmethod remains the same):

Approach One (Full deserialization and serialization) ...

    private Pony(SerializationInfo info, StreamingContext context) {

      foreach (SerializationEntry entry in info) {
        switch (entry.Name) {
          case "Id":
            this.Id = (int)entry.Value;
            break;
          case "Name":
            this.Name = (string)entry.Value;
            break;
          case "BFF":
            this.BFF = (Pony)entry.Value;
            break;
        }
      }
    }

    public void GetObjectData(SerializationInfo info, StreamingContext ontext) {
      info.AddValue("Id", Id);
      info.AddValue("Name", Name);
      info.AddValue("BFF", BFF);
    }
  }

...

Approach 2 (Recursive object - Id only) :

...

private Pony(SerializationInfo info, StreamingContext context) {

      foreach (SerializationEntry entry in info) {
        switch (entry.Name) {
          case "Id":
            this.Id = (int)entry.Value;
            break;
          case "Name":
            this.Name = (string)entry.Value;
            break;
          case "BFF.Id":
            var bffId = (int)entry.Value;
            this.BFF = GetPonyById(bffId); // You have to implement this
            break;
        }
      }
    }

    public void GetObjectData(SerializationInfo info, StreamingContext ontext) {
      info.AddValue("Id", Id);
      info.AddValue("Name", Name);
      info.AddValue("BFF.Id", BFF.Id);
    }

...