using System;

using System.Text.Json;

public class Program

{

    public static void Main()

    {

        var values = new object[]

        {

            new A { PropertyA = "A value" },

            new B { PropertyB = "B value" },

        };

        Console.WriteLine(JsonSerializer.Serialize(values));

    }

    public class A

    {

        public string PropertyA { get; set; }

    }

    public class B

    {

        public string PropertyB { get; set; }

    }

}