What's up with the [OptionalField] Attribute?

Question

As I understand it I have to adorn a new member in a newer version of my class with the [OptionalField] Attribute when I deserialize an older version of my class that lacks this newer member.

However, the code below throws no exception while the InnerTranslator property was added after serializing the class. I check for the property to be null in the onDeserialization method (which confirms that it was not serialized),but I would have expected the code to throw an exception because of that. Is the [OptionalField] Attribute itself optional?

class Program
{
    static void Main(string[] args)
    {
        var listcol = new SortedList<string,string>
        {
            {"Estados Unidos", "United States"},
            {"Canadá", "Canada"},
            {"España", "Spain"}
        };
        var translator = new CountryTranslator(listcol);
        using (var file_stream=new FileStream("translator.bin",FileMode.Open))
        {
            var formatter = new BinaryFormatter();
            translator = formatter.Deserialize(file_stream) as CountryTranslator;
            file_stream.Close();
        }
        Console.ReadLine();
    }
}

[Serializable]
internal class CountryTranslator:IDeserializationCallback
{
    public int Count { get; set; }

    public CountryTranslator(SortedList<string,string> sorted_list)
    {
        this.country_list = sorted_list;
        inner_translator = new List<string> {"one", "two"};
    }
    //[OptionalField]
    private List<string> inner_translator;
    public List<string> InnerTranslator
    {
        get { return inner_translator; }
        set { inner_translator = value; }
    }

    private SortedList<string, string> country_list;

    public void OnDeserialization(object sender)
    {
        Debug.Assert(inner_translator == null);
        Count=country_list.Count;
    }
}

Answer 1

BinaryFormatter is, at the best of times, very brittle if you change things. Not least, there are huge problems with automatically implemented properties, obfuscaction, renaming, strong naming, etc.

As I recall, some of the rules about [OptionalField] changed just before it was released; the version tolerant thing didn't really work out as easily as had been planned, I expect.

My advice: if you want version tolerant serialization (i.e. you can serialize it today and deserialize it with the next version of your app), then don't use BinaryFormatter; this is (IMO) only suitable for passing data between the same version (remoting, AppDomains, etc).

For work between versions, I recommend contract-based serialization; things like XmlSerializer and DataContractSerializer (.NET 3.0), or for binary - protobuf-net or similar tools. All of these are much better at version tolerance (indeed, you don't even need to deserialize it into the same Type); plus they can be used between platforms - so you can serialize in .NET and deserialize in java/C++/etc.