Json – Deserializing private members

I have the following code, taken from https://learn.microsoft.com/En-Us/dotnet/standard/serialization/system-text-json/custom-contracts#example-serialize-private-fields. It works fine to serialize the fields of Brains.Brain, but when deserializing, the instance created has (a, b, c, d) = (2, 0, 4, 4) when it should be (2, 3, 4, 7). I’ve tried finding more on how to use a Json constructor, but haven’t been able to find much.

Would someone be able to direct me to if it is on the microsoft documentation (although I haven’t found it); somewhere else where it is documented; or some example code on how I should be using it or something else which solves this problem.

using System.IO;
using System.Linq;
using System.Text.Json;
//
using System.Reflection;
using System.Text.Json.Serialization;
using System.Text.Json.Serialization.Metadata;
using static System.Runtime.InteropServices.JavaScript.JSType;

namespace BinaryFiles
{
    class Program
    {
        static void Main(string[] args)
        {

            Brains.Brain b = new(3);
            Type btype = typeof(Brains).GetNestedTypes().Where(brain => (brain.GetInterface("IBrain") != null)).ToArray()[0];
            Loader.Save("test", b, btype);
            b.test();

            IBrain newB = Loader.Load("test", btype);
            newB.test();

        }
    }

    internal interface IBrain
    {
        public void test();
    }
    class Brains {
        [JsonIncludePrivateFields]
        public class Brain : IBrain 
        {
            private int a = 2;
            private int b;
            public int c = 4;
            public int d;

            /*
            [JsonConstructor]
            public Brain(int a, int b, int c, int d) =>
            (this.a, this.b, this.c, this.d) = (a, b, c, d);
            */

            public Brain(int B)
            {
                this.b = B;
                this.d = B + 4;
            }
            public void test()
            {
                Console.WriteLine($"{a} {b} {c} {d}");
            }
        }
    }

    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct)]
    public class JsonIncludePrivateFieldsAttribute : Attribute { }

    internal static class Loader
    {
        static void AddPrivateFieldsModifier(JsonTypeInfo jsonTypeInfo)
        {
            if (jsonTypeInfo.Kind != JsonTypeInfoKind.Object)
                return;

            if (!jsonTypeInfo.Type.IsDefined(typeof(JsonIncludePrivateFieldsAttribute), inherit: false))
                return;

            foreach (FieldInfo field in jsonTypeInfo.Type.GetFields(BindingFlags.Instance | BindingFlags.NonPublic))
            {
                JsonPropertyInfo jsonPropertyInfo = jsonTypeInfo.CreateJsonPropertyInfo(field.FieldType, field.Name);
                jsonPropertyInfo.Get = field.GetValue;
                jsonPropertyInfo.Set = field.SetValue;

                jsonTypeInfo.Properties.Add(jsonPropertyInfo);
            }
        }

        public static void Save(string fileName, IBrain brain, Type brainType)
        {
            var options = new JsonSerializerOptions
            {
                IncludeFields = true,
                WriteIndented = true,
                TypeInfoResolver = new DefaultJsonTypeInfoResolver
                {
                    Modifiers = { AddPrivateFieldsModifier }
                }
            };
            Console.WriteLine(JsonSerializer.Serialize(brain, brainType, options));
            File.WriteAllText(fileName + ".json", JsonSerializer.Serialize(brain, options));
        }

        public static IBrain Load(string fileName, Type brainType)
        {
            var options = new JsonSerializerOptions
            {
                IncludeFields = true,
                WriteIndented = true,
                TypeInfoResolver = new DefaultJsonTypeInfoResolver
                {
                    Modifiers = { AddPrivateFieldsModifier }
                }
            };
            return (IBrain)JsonSerializer.Deserialize(File.ReadAllText(fileName + ".json"), brainType, options);
        }
    }
}```