Add a property to Json on serialize

AddPropertyToJsonOnSerialize.cs

using System;
using System.Collections.Generic;
using Newtonsoft.Json;
using Newtonsoft.Json.Serialization;
using NUnit.Framework;

namespace AddToJsonPoco
{
    [TestFixture]
    public class AddPropertyToJsonOnSerialize
    {
        [Test]
        public void Without()
        {
            var model = new Model {id = 123};
            var json = JsonConvert.SerializeObject(model);
            Assert.AreEqual("{\"id\":123}", json);
        }

        private readonly JsonSerializerSettings _jsonSerializerSettings = new JsonSerializerSettings
        {
            ContractResolver = new AppendProperties(
                new JsonProperty
                {
                    PropertyName = "I do not exist",
                    Readable = true,
                    Writable = false,
                    PropertyType = typeof (string),
                    ValueProvider = new FixedValueProvider("hello!")
                })
        };

        [Test]
        public void With()
        {
            var model = new Model {id = 123};

            var json = JsonConvert.SerializeObject(model, _jsonSerializerSettings);

            Assert.AreEqual("{\"id\":123,\"I do not exist\":\"hello!\"}", json);
        }
    }

    public class Model
    {
        public int id;
    }

    public sealed class AppendProperties : DefaultContractResolver
    {
        private readonly JsonProperty[] _jsonProperties;

        public AppendProperties(params JsonProperty[] jsonProperties)
        {
            _jsonProperties = jsonProperties;
        }

        protected override IList<JsonProperty> CreateProperties(Type type, MemberSerialization memberSerialization)
        {
            var res = base.CreateProperties(type, memberSerialization);
            foreach (var jsonProperty in _jsonProperties)
                res.Add(jsonProperty);
            return res;
        }
    }

    public sealed class FixedValueProvider : IValueProvider
    {
        private readonly object _value;

        public FixedValueProvider(object value)
        {
            _value = value;
        }

        public void SetValue(object target, object value)
        {
            throw new NotImplementedException();
        }

        public object GetValue(object target)
        {
            return _value;
        }
    }
}