IDictionary<string, object> Serialize(object obj, JavaScriptSerializer serializer)

public override IEnumerable<Type> SupportedTypes
{
    get
    {
        var list = new List<Type>{ typeof(Foo), typeof(Bar)...};
        return list.AsReadOnly();
    }
}

public IDictionary<string, object> GetNonNullProertyValues(object obj)
{
    var dictionary = new Dictionary<string, object>();

    foreach (var property in obj.GetType().GetProperties())
    {
        var propertyValue = property.GetValue(obj, null);
        if (propertyValue != null)
        {
            dictionary.Add(property.Name, propertyValue);
        }
    }

    return dictionary;
}

using System.IO;
 using System.Runtime.Serialization.Json;

    public static class JsonExtensions
    {
        public static string ToJson<T>(this T instance)
        {
            var serializer = new DataContractJsonSerializer(typeof(T));
            using (MemoryStream memoryStream = new MemoryStream())
            {
                serializer.WriteObject(memoryStream, instance);

                memoryStream.Flush();
                memoryStream.Position = 0;

                using (var reader = new StreamReader(memoryStream))
                {
                    return reader.ReadToEnd();
                }
            }
        }

        public static T FromJson<T>(this string serialized)
        {
            var serializer = new DataContractJsonSerializer(typeof(T));
            using (MemoryStream memoryStream = new MemoryStream())
            {
                using (StreamWriter writer = new StreamWriter(memoryStream))
                {
                    writer.Write(serialized);
                    writer.Flush();

                    memoryStream.Position = 0;

                    return (T)serializer.ReadObject(memoryStream);
                }
            }
        }
    }