using System;

using System.Collections.Generic;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        List<Base> list = new List<Base>();

        list.Add( new Base { Id = 1, ParentId = -1 });

        list.Add( new Foo { Id = 2, ParentId = 1 });

        string json = JsonConvert.SerializeObject(list, Formatting.Indented);

​

        Console.WriteLine(json);

    }

}

​

class Base

{

    public int Id { get; set; }

    public int ParentId { get; set; }

​

    public virtual bool ShouldSerializeParentId()

    {

        return false;

    }

}

​

class Foo : Base

{

    public override bool ShouldSerializeParentId()

    {

        return true;

    }

}

​