using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.IO;

using System.Globalization;

using System.Numerics;

​

using System.Text.Json;

using System.Text.Json.Serialization;

​

using NUnit.Framework; // Throws NUnit.Framework.AssertionException

​

//https://stackoverflow.com/questions/64788895/serialising-biginteger-using-system-text-json

​

    public class BigIntegerConverter : JsonConverter<BigInteger>

    {

        public override BigInteger Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)

        {

            if (reader.TokenType != JsonTokenType.Number)

                throw new JsonException(string.Format("Found token {0} but expected token {1}", reader.TokenType, JsonTokenType.Number ));

            using var doc = JsonDocument.ParseValue(ref reader);

            return BigInteger.Parse(doc.RootElement.GetRawText(), NumberFormatInfo.InvariantInfo);

        }

​

        public override void Write(Utf8JsonWriter writer, BigInteger value, JsonSerializerOptions options) =>

            writer.WriteRawValue(value.ToString(NumberFormatInfo.InvariantInfo), false);

    }

​

    class TestClass

    {

        public static void Test()

        {

            var options = new JsonSerializerOptions();

            options.Converters.Add(new BigIntegerConverter());

            var s = JsonSerializer.Serialize(new {foo = new BigInteger(ulong.MaxValue) + 1}, options);          

            Assert.AreEqual(@"{""foo"":18446744073709551616}", s);

            Console.WriteLine(s);

            Test(0);

            var i = new BigInteger(ulong.MaxValue) + 1;

            Test(i);

            Test(i * i * i);

            Test(i * i * i * i * i * i);

            Test(-i * i * i * i * i * i);

            var i2 = -i * i * i * i * i * i;

            var i3 = i2 * i2 * i2 * i2 * i * i;

            Test(i3);

            var i4 = i3^10 + i2;

            Test(-i4*i4*i4*i4*i4*i4*i4*i4*i4*i4*i4*i4);

        }

        public static void Test(BigInteger i)

        {           

            var options = new JsonSerializerOptions

            {

                Converters = { new BigIntegerConverter() },

            };

            var json = JsonSerializer.Serialize(i, options);            

            Console.WriteLine(json);

            var i2 = JsonSerializer.Deserialize<BigInteger>(json, options);

            Assert.IsTrue(i == i2);

        }   

    }

​

    public class Program

    {

        public static void Main()

        {

            Console.WriteLine("Environment version: {0} ({1})", System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription , Environment.Version);

            Console.WriteLine("System.Text.Json version: " + typeof(JsonSerializer).Assembly.FullName);

            Console.WriteLine();

​

            try

            {

                TestClass.Test();

            }

            catch (Exception ex)

            {

                Console.WriteLine("Failed with unhandled exception: ");

                Console.WriteLine(ex);

                throw;

            }

        }

    }

​