XmlSerializer: keep unknown elements

Question

I have a class that is serialized into/deserialized from XML and stored in/restored from a file:

public class Customer
{
    public string FirstName;
    public string LastName;

    public Customer()
    {
    }

    public Customer(string firstName, string lastName)
    {
        FirstName = firstName;
        LastName = lastName;
    }

    public static Customer Load(TextReader reader)
    {
        XmlSerializer deserializer = new XmlSerializer(typeof(Customer));
        return (Customer)deserializer.Deserialize(reader);
    }

    public void Save(TextWriter writer)
    {
        XmlSerializer serializer = new XmlSerializer(GetType());
        serializer.Serialize(writer, this);
    }
}

In a newer version of this class I added a new property

public string MiddleName;

It is a common use case that a user has installed both the old and new version of my program. They both read and write the same serialized file. When the new version writes the file, all three properties (FirstName, LastName, MiddleName) are written. The old program reads the file but omits the unknown element MiddleName. It saves the file without MiddleName, so it's value is lost for the newer program.

Is there a way to store the original XML when deserializing and 'merge' the unknown elements back in when serializing? The old program would ignore unknown elements but write them back into the file so they are not lost for the new program.

Answer 1

Can't test it right now as I'm on Mac OS X, but XmlAnyElement should work:

[XmlAnyElement]
public XmlElement[] Unsupported { get; set; }

Answer 2

XMLSerializer serializes. If the info is not available how could it serialize that?

What you could do is update the xmlfile, instead of serializing and overwriting it. It is extra work because you will have to traverse the file yourself but it allows you to achieve what you want.

How To Modify and Save XML with the XmlDocument Class in the .NET Framework SDK