JsonSerialization

#define MANUAL_SERIALIALIZATION
using System.Text;
using System;
using System.IO;
using System.Linq;
using System.Collections.Generic;
using System.Diagnostics;
using System.Text.Json;

public class WeatherForecast
{
    public DateTime Date { get; set; }

    public int TemperatureC { get; set; }

    public int TemperatureF => 32 + (int)(TemperatureC / 0.5556);

    public string Summary { get; set; }
}

class Program
{
    private static readonly string[] Summaries = new[]
    {
        "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching"
    };

    public static IEnumerable<WeatherForecast> Get()
    {
        var rng = new Random();
        return Enumerable.Range(1, 5).Select(index => new WeatherForecast
        {
            Date = DateTime.Now.AddDays(index),
            TemperatureC = rng.Next(-20, 55),
            Summary = Summaries[rng.Next(Summaries.Length)]
        })
        .ToArray();
    }

    static void Main(string[] args)
    {
        var result = Get();
        var sw = new Stopwatch();
        sw.Start();
#if MANUAL_SERIALIALIZATION
        var ms = new MemoryStream();
        using (var writer = new Utf8JsonWriter(ms))
        {
            writer.WriteStartArray();
            foreach (var w in result)
            {
                writer.WriteStartObject();
                writer.WriteString("Date", w.Date);
                writer.WriteNumber("TemperatureC", w.TemperatureC);
                writer.WriteNumber("TemperatureF", w.TemperatureF);
                writer.WriteString("Summary", w.Summary);
                writer.WriteEndObject();
            }
            writer.WriteEndArray();
        }
#else
        JsonSerializer.SerializeToUtf8Bytes(result);
#endif
        sw.Stop();
        Console.WriteLine(sw.ElapsedMilliseconds);
    }
}