[DataContract, Serializable, KnownType(typeof(B))]
    public class A
    {
        [DataMember]
        int x = 42;
    }

    [DataContract, Serializable]
    public class B : A
    {
        [DataMember]
        int y = 24;
    }


    class Program
    {

        public static string SerialiseAs<TResult>(TResult input)
        {
            DataContractJsonSerializer ser = new DataContractJsonSerializer(typeof(TResult));
            MemoryStream stream = new MemoryStream();
            ser.WriteObject(stream, input);
            stream.Position = 0;
            StreamReader reader = new StreamReader(stream);
            return reader.ReadToEnd();
        }

        static void Main(string[] args)
        {
            Console.WriteLine(SerialiseAs<A>(new B()));
        }
    }