using System;

using System.Collections.Generic;

using System.IO;

using System.Text.Json;

using System.Text;

public class Program

{

    public static void Main()

    {

        var options = new JsonWriterOptions

        {

            Indented = true

        };

        using(var stream = new MemoryStream())

        {

            using(var writer = new Utf8JsonWriter(stream, options))

            {

                writer.WriteStartObject();

                writer.WriteString("hello", "world");

                writer.WriteEndObject();

            }

            Console.WriteLine(Encoding.UTF8.GetString(stream.ToArray()));

        }

        // or

        var dict = new Dictionary<string, string>();

        dict["hello"] = "world";

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

    }

}