Custom JsonConverter

public class KeysJsonConverter : JsonConverter
{
    private readonly Type[] _types;

    public KeysJsonConverter(params Type[] types)
    {
        _types = types;
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        JToken t = JToken.FromObject(value);

        if (t.Type != JTokenType.Object)
        {
            t.WriteTo(writer);
        }
        else
        {
            JObject o = (JObject)t;
            IList<string> propertyNames = o.Properties().Select(p => p.Name).ToList();

            o.AddFirst(new JProperty("Keys", new JArray(propertyNames)));

            o.WriteTo(writer);
        }
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        throw new NotImplementedException("Unnecessary because CanRead is false. The type will skip the converter.");
    }

    public override bool CanRead
    {
        get { return false; }
    }

    public override bool CanConvert(Type objectType)
    {
        return _types.Any(t => t == objectType);
    }
}

public class Employee
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public IList<string> Roles { get; set; }
}

Employee employee = new Employee
{
    FirstName = "James",
    LastName = "Newton-King",
    Roles = new List<string>
    {
        "Admin"
    }
};

string json = JsonConvert.SerializeObject(employee, Formatting.Indented, new KeysJsonConverter(typeof(Employee)));

Console.WriteLine(json);
// {
//   "Keys": [
//     "FirstName",
//     "LastName",
//     "Roles"
//   ],
//   "FirstName": "James",
//   "LastName": "Newton-King",
//   "Roles": [
//     "Admin"
//   ]
// }

Employee newEmployee = JsonConvert.DeserializeObject<Employee>(json, new KeysJsonConverter(typeof(Employee)));

Console.WriteLine(newEmployee.FirstName);
// James