using System;

using System.Reflection;

using Newtonsoft.Json;

using Newtonsoft.Json.Serialization;

public class Program

{

    public static void Main()

    {

        var obj = new DerivedClass { Field1 = "aaa", Field2 = "bbb" };

        var json = JsonConvert.SerializeObject(obj, new JsonSerializerSettings { ContractResolver = new NoDerivedContractResolver() });

        Console.WriteLine(json);

    }

​

    public class NoDerivedContractResolver : DefaultContractResolver

    {

        protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)

        {

            JsonProperty property = base.CreateProperty(member, memberSerialization);

            property.ShouldSerialize = _ => property.DeclaringType != typeof(DerivedClass);

            return property;

        }

    }

​

    public class BaseClass

    {

        public string Field1 { get; set; }

    }

​

    public class DerivedClass : BaseClass

    {

        public string Field2 { get; set; }

    }

}