Question

Are these fields set to default values like when a new instance is created? Can I create custom code to set these fields to new values upon deserialization?

Answer 1

Note: all of this is specific to BinaryFormatter:

By default, they are ignored completely; they will have their type-default values, i.e. a zero-value / null-value.

If you implement custom serialization (ISerializable), then [NonSerializable] doesn’t apply, and you can do what you want, but most people don’t want to have to do this. However, you can also implement IDeserializationCallback, which provides an ideal opportunity to initialize such fields:

[Serializable]
class Foo : IDeserializationCallback
{
    // ... not shown
    void IDeserializationCallback.OnDeserialization(object sender)
    {
        // init your [NonSerialized] fields here
    }
}

Note that other serializers have different implementations for serialization callbacks, some of which are also supported by BinaryFormatter, for example:

[Serializable]
class Foo
{
    // ... not shown
    [OnDeserializing]
    private void AnyMethodName(StreamingContext c)
    {
        // init your [NonSerialized] fields here
    }
}

The attribute-based callbacks provide more opportunities to inject code at specific points, and are usually preferred. There are 4: [OnDeserializing], [OnDeserialized], [OnSerializing] and [OnSerialized].