using System;

using System.Text.Json;

​

​

public class Program

{

    public static void Main()

    {

        dynamic w = new ExpandoObject() { Date = DateTime.Now, Item1 = 30 };

        w.Item2 = 123;

        Console.WriteLine(JsonSerializer.Serialize<dynamic>(w));

    }

    class ExpandoObject

    {

        public DateTimeOffset Date { get; set; }

        public int Item1 { get; set; }

        public int Item2 { get; set; }

    }

}