using System;

using Newtonsoft.Json;                  

public class Program

{

    public static void Main()

    {

        Base[] arrayOfBase = { new DerivedA(){baseInt=2,derivedString = "a"}, new DerivedB(){baseInt=3,derivedFloat=3.5F} };

        var serializedData = JsonConvert.SerializeObject(arrayOfBase, Newtonsoft.Json.Formatting.Indented);

        Console.WriteLine(serializedData);

    }

}

​

[System.Serializable]

public abstract class Base {

    public int baseInt;

}

​

[System.Serializable]

public class DerivedA : Base {

    public string derivedString;

}

​

[System.Serializable]

public class DerivedB : Base {

    public float derivedFloat;

}

​

​