How do I deserialize old data for a type that has changed?

Question

I have data that has been stored using binary serialization for the following class:

[Serializable]
public abstract class BaseBusinessObject
{
    private NameValueCollection _fieldErrors = new NameValueCollection();

    protected virtual NameValueCollection FieldErrors
    {
        get { return _fieldErrors; }
        set { _fieldErrors = value; }
    }

    ...
}

At some point, the class was changed to this:

[Serializable]
public abstract class BaseBusinessObject
{
    private Dictionary<string, string> _fieldErrors = new Dictionary<string, string>();

    protected virtual Dictionary<string, string> FieldErrors
    {
        get { return _fieldErrors; }
        set { _fieldErrors = value; }
    }

    ...
}

This is causing issues deserializing old data.

My first thought was to implement ISerializable, but this class has numerous properties as well as hundreds of inheriting classes that I would have to implement this for as well.

I would like to either change the old data to match the current structure during deserialization or have a clean way of upgrading the old data.

Answer 1

Add the new _fieldErrors under a different name, say _fieldErrors2, and make it [Optional]. Then implement an [OnDeserialized] Method that copies the data from _fieldErrors to _fieldErrors2 (if present), and clears _fieldErrors.

Answer 2

If the data is only used internally, my first thought would be to write some simple throw-away code to de-serialize your binary data using the old "NameValueCollection", map it to a Dictionnary and re-serialize it. Even if it'll take a few days to process all the data, it doesn't seem worth it to implement a patch on your new code to support the old data.

Even if it's not only used internally, an importer seems like the simplest way to go.