マスターデータに型つける.cs

CompositeResolver.RegisterAndSetAsDefault(new IJsonFormatter[]
{
    new IntValueFormatter<Gold>(x => new Gold(x)),
    new IDFormatter<StageID>(x => new StageID(x)),
}, new[]
{
    StandardResolver.Default,
});

// -----

private sealed class IntValueFormatter<T> : IJsonFormatter<T> where T : IIntValue
{
    private readonly Func<int, T> factory;

    public IntValueFormatter(Func<int, T> factory)
    {
        this.factory = factory;
    }

    public void Serialize(ref JsonWriter writer, T value, IJsonFormatterResolver formatterResolver)
    {
        throw new NotImplementedException();
    }

    public T Deserialize(ref JsonReader reader, IJsonFormatterResolver formatterResolver)
    {
        return factory(reader.ReadInt32());
    }
}

private sealed class IDFormatter<T> : IJsonFormatter<T> where T : ITypeID
{
    private readonly Func<uint, T> factory;

    public IDFormatter(Func<uint, T> factory)
    {
        this.factory = factory;
    }

    public void Serialize(ref JsonWriter writer, T value, IJsonFormatterResolver formatterResolver)
    {
        throw new NotImplementedException();
    }

    public T Deserialize(ref JsonReader reader, IJsonFormatterResolver formatterResolver)
    {
        var stringID = reader.ReadString();
        return factory(StringToID(stringID));
    }
}