using System;

using System.Collections.Specialized;

using System.Text.Json;

public class Program

{

    public static void Main()

    {

        OrderedDictionary dict = new();

        dict.Add("1", new Test { Id = 1, Value = "value1" });

        dict.Add("2", new Test { Id = 2, Value = "valueX" });

        dict.Add("3", new Test { Id = 3, Value = "value3" });

        var key = "2";

        // careful, the key cannot be integer otherwise it will use the dict GetByIndex instead of GetByKey

        if (dict.Contains(key)) {

            (dict[key] as Test).Value = "value2";

        }

        var json = JsonSerializer.Serialize(dict, new JsonSerializerOptions() { WriteIndented = true });

        // var json = JsonSerializer.Serialize(dict); // indentation is not required

        Console.WriteLine(json);

    }

    public class Test {

        public int Id { get; set; }

        public string Value { get; set; }

    }

}