Serialization support

[LWEdslev]

Currently serialization using System.Xml.Serialization does not work as I expected.
The following works:

using System.Xml.Serialization;
using newtype;

var xml = new XmlSerializer(typeof(Order));
var order = new Order { Contract = "CTR-002" };
using var sw = new StringWriter();
xml.Serialize(sw, order);
Console.WriteLine($"XML: {sw}");

public class Order { public string Contract { get; set; } }

but using newtype it did not:

using System.Xml.Serialization;
using newtype;

var xml = new XmlSerializer(typeof(Order));
var order = new Order { Contract = new ContractId("CTR-002") };
using var sw = new StringWriter();
xml.Serialize(sw, order);
Console.WriteLine($"XML: {sw}");
// <Id /> — empty, value not serialized

public class Order { public ContractId Contract { get; set; } }

[newtype<string>]
public readonly partial struct ContractId;

I suspect similar problems exist for other serializers.

[thygrrr]

Great bug report, this is an issue I have not thought about (because my serialization approach is different in most of my use cases).

Your issue is valid, but solving it may be considerably difficult. I'm looking into it.