Preciso adicionar KnownType ao código abaixo para serializar com sucesso. Quando faço isso, o JSON gerado é o seguinte:

JSON form of Adult with 1 child: {"age":42,"name":"John","children":[{"__type":"
Child:#TestJson","age":4,"name":"Jane","fingers":10}]}

Como faço para que não inclua o "__type": "Child: #TestJson"? Retornamos centenas desses elementos em algumas consultas e esse texto extra será adicionado.

Código completo:

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

namespace TestJson
{
    class Program
    {
        static void Main(string[] args)
        {
            Adult parent = new Adult {name = "John", age = 42};

            MemoryStream stream1 = new MemoryStream();
            DataContractJsonSerializer ser = new DataContractJsonSerializer(typeof(Adult));
            ser.WriteObject(stream1, parent);

            stream1.Position = 0;
            StreamReader sr = new StreamReader(stream1);
            Console.Write("JSON form of Adult with no children: ");
            Console.WriteLine(sr.ReadToEnd());


            Child child = new Child { name = "Jane", age = 4, fingers=10 };

            stream1 = new MemoryStream();
            ser = new DataContractJsonSerializer(typeof(Child));
            ser.WriteObject(stream1, child);

            stream1.Position = 0;
            sr = new StreamReader(stream1);
            Console.Write("JSON form of Child with no parent: ");
            Console.WriteLine(sr.ReadToEnd());


            // now connect the two
            parent.children.Add(child);

            stream1 = new MemoryStream();
            ser = new DataContractJsonSerializer(typeof(Adult));
            ser.WriteObject(stream1, parent);

            stream1.Position = 0;
            sr = new StreamReader(stream1);
            Console.Write("JSON form of Adult with 1 child: ");
            Console.WriteLine(sr.ReadToEnd());
        }
    }

    [DataContract]
    [KnownType(typeof(Adult))]
    [KnownType(typeof(Child))]
    class Person
    {
        [DataMember]
        internal string name;

        [DataMember]
        internal int age;
    }

    [DataContract]
    class Adult : Person
    {
        [DataMember] 
        internal List<Person> children = new List<Person>();
    }

    [DataContract]
    class Child : Person
    {
        [DataMember]
        internal int fingers;
    }
}