using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Serialization;
using System.Text;
using System.Threading.Tasks;
using System.IO;

namespace ConsoleApplication1
{
    [DataContract]
    [KnownType(typeof(B))]
    public class A
    {
        [DataMember]
        public Dictionary<string, object> Dict { get; set; }
    }

    [DataContract]
    public class B
    {
        [DataMember]
        public string X { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var obj = new A
            {
                Dict = new Dictionary<string, object>
                {
                    { "a", new B { X = "123" } }
                }
            };

            new DataContractSerializer(typeof(A)).WriteObject(Stream.Null, obj);
        }
    }
}