using System;

using System.Collections.Generic;

using System.Text.Json;

using System.Text.Json.Serialization;

public class Program

{

    public static void Main()

    {

        byte[] arr = { 1, 2, 3};

        string serialized = JsonSerializer.Serialize(arr);

        Console.WriteLine(serialized);

        // With converter.

        JsonSerializerOptions options = new() { Converters = { new ByteArrayConverter() } };

        serialized = JsonSerializer.Serialize(arr, options);

        Console.WriteLine(serialized);

​

        // Test round-trip.

        arr = JsonSerializer.Deserialize<byte[]>(serialized, options);

        Console.WriteLine(serialized == JsonSerializer.Serialize(arr, options));        

    }

    public class ByteArrayConverter : JsonConverter<byte[]>

    {

        public override byte[] Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)

        {

            // Can be optimized (e.g. by parsing with reader directly to avoid serializer overhead).

            return JsonSerializer.Deserialize<List<byte>>(ref reader, options)?.ToArray();

        }

        public override void Write(Utf8JsonWriter writer, byte[] value, JsonSerializerOptions options)

        {

            if (value == null)

            {

                writer.WriteNullValue();

                return;

            }

            writer.WriteStartArray();

            for (int i = 0; i < value.Length; i++)

            {

                writer.WriteNumberValue(value[i]);

            }

            writer.WriteEndArray();

        }

    }

}