Quickduck

I had always thought that when an object got deserialized (using the default serialization support), the basic process was:

1. A new object is created using the object’s default constructor
2. The object’s properties are populated via reflection from the object’s serialized stream

Turns out that’s not true – the constructor is never called! This was a problem for me as I was initializing my NonSerialized variables in the constructor, but after the object was Deserialized and I tried to access them, I was getting a NullReferenceException:

[Serializable]
public class Person {
    private string name;
    [NonSerialized] private IList friends;

    public string Name {
        get { return name; }
        set { name = value; }
    }

    public void AddFriend(string name) {
        friends.Add(name);    // throws NRE after deserialization
    }

    public Person() {
        friends = new List();
    }
}

Turns out that the correct way of doing something like this is to use the OnDeserialized attribute on an initialization method. The attribute requires that the method signature takes a single parameter – a StreamingContext:

[Serializable]
public class Person {
    private string name;
    [NonSerialized] private IList friends;

    public string Name {
        get { return name; }
        set { name = value; }
    }

    public void AddFriend(string name) {
        friends.Add(name);    // no more NRE after deserialization
    }

    [OnDeserialized]
    private void Init(StreamingContext context) {
        friends = new List();
    }

    public Person() {
        Init(new StreamingContext());
    }
}

I’m not sure if this is technically the best way of achieving this, but it works, and besides creating an empty StreamingContext in the constructor (StreamingContext is a struct so you can’t use null), it feels ok to me. Any thoughts?