[MessagePackObject]
public class MyClass {
    [Key(0)]
    public int Age { get; set; }
    [Key(1)]
    public string FirstName { get; set; }
    [Key(2)]
    public string LastName { get; set; }

    static void Main(string[] args) {
        var mc = new MyClass {
            Age = 99,
            FirstName = "hoge",
            LastName = "huga",
        };

        byte[] bytes = MessagePackSerializer.Serialize(mc);
        MyClass mc2 = MessagePackSerializer.Deserialize<MyClass>(bytes);

        // [99,"hoge","huga"]
        var json = MessagePackSerializer.ConvertToJson(bytes);
        Console.WriteLine(json);
    }
}