ExpandoObject recursive extension method for JSON serialization

DynamicObjectHelper

/*
*Original version posted by TimDog in a StackOverflow question: http://stackoverflow.com/questions/5156664/how-to-flatten-an-expandoobject-returned-via-jsonresult-in-asp-net-mvc
*/

  public static class DynamicObjectHelper
    {

        public static string Flatten(this ExpandoObject expando)
        {
            return FlattenImplementation(expando);
        }


        private static string FlattenImplementation(ExpandoObject expando)
        {
            StringBuilder sb = new StringBuilder();
            List<string> contents = new List<string>();
            var d = expando as IDictionary<string, object>;
            sb.Append("{");

            JsonSerializerSettings settings = new JsonSerializerSettings()
            {
                NullValueHandling = NullValueHandling.Ignore,
            };

            foreach (KeyValuePair<string, object> kvp in d)
            {

                if (kvp.Value is ExpandoObject)
                {
                    contents.Add(String.Format("\"{0}\": {1}", kvp.Key, FlattenImplementation(kvp.Value as ExpandoObject)));
                }
                else
                {
                    contents.Add(String.Format("\"{0}\": {1}", kvp.Key, JsonConvert.SerializeObject(kvp.Value, settings)));
                }


            }
            sb.Append(String.Join(",", contents.ToArray()));

            sb.Append("}");

            return sb.ToString();
        }

    }

Original version posted by TimDog in a StackOverflow question: http://stackoverflow.com/questions/5156664/how-to-flatten-an-expandoobject-returned-via-jsonresult-in-asp-net-mvc