preserving references + custom converters

preserve_with_custom_converter.cs

// Assuming that you wrote a custom converter for `Company` and you don't want to manually serialize the `Supervisor`, which is an `Employee`. 
// you want to delegate that to the Serializer and you also want to preserve the references that you have already saved.

using System;
using System.Collections.Generic;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace ConsoleApp4
{
    class Program
    {
        public class Employee
        {
            public string Name { get; set; }
            public Employee Manager { get; set; }
            public List<Employee> DirectReports { get; set; }
            public Company Company { get; set; }
        }

        public class Company
        {
            public string Name { get; set; }
            public Employee Supervisor { get; set; }
        }

        static void Main(string[] args)
        {
            Employee tyler = new()
            {
                Name = "Tyler Stein"
            };

            Employee adrian = new()
            {
                Name = "Adrian King"
            };

            Company contoso = new()
            {
                Name = "Contoso",
                Supervisor = tyler
            };

            tyler.DirectReports = new List<Employee> { adrian };
            adrian.Manager = tyler;

            tyler.Company = contoso;
            adrian.Company = contoso;

            var options = new JsonSerializerOptions();

            options.Converters.Add(new CompanyConverter());
            var myReferenceHandler = new MyReferenceHandler();
            options.ReferenceHandler = myReferenceHandler;
            options.DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull;
            options.WriteIndented = true;

            string str = JsonSerializer.Serialize(tyler, options);

            // Always reset after you are done serializing in order to avoid out of bounds memory growth in the resolver.
            myReferenceHandler.Reset();
            
            Console.WriteLine(str); 
            // Output:
            //{
            //  "$id": "1",
            //  "Name": "Tyler Stein",
            //  "DirectReports": {
            //    "$id": "2",
            //    "$values": [
            //      {
            //        "$id": "3",
            //        "Name": "Adrian King",
            //        "Manager": {
            //          "$ref": "1"
            //        },
            //        "Company": {
            //          "Name": "Contoso",
            //          "Supervisor": {
            //            "$ref": "1"
            //          }
            //        }
            //      }
            //    ]
            //  },
            //  "Company": {
            //    "Name": "Contoso",
            //    "Supervisor": {
            //      "$ref": "1"
            //    }
            //  }
            //}
        }

        class CompanyConverter : JsonConverter<Company>
        {
            public override Company Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
            {
                throw new NotImplementedException();
            }

            public override void Write(Utf8JsonWriter writer, Company value, JsonSerializerOptions options)
            {
                writer.WriteStartObject();

                writer.WriteString("Name", value.Name);

                writer.WritePropertyName("Supervisor");
                JsonSerializer.Serialize(writer, value.Supervisor, options);
                
                writer.WriteEndObject();
            }
        }

        class MyReferenceHandler : ReferenceHandler
        {
            private ReferenceResolver _rootedResolver;
            public MyReferenceHandler() => Reset();
            public override ReferenceResolver CreateResolver() => _rootedResolver;
            public void Reset() => _rootedResolver = new MyReferenceResolver();

            class MyReferenceResolver : ReferenceResolver
            {
                private uint _referenceCount;
                private readonly Dictionary<string, object> _referenceIdToObjectMap = new Dictionary<string, object>();
                private readonly Dictionary<object, string> _objectToReferenceIdMap = new Dictionary<object, string>(ReferenceEqualityComparer.Instance);

                public override void AddReference(string referenceId, object value)
                {
                    if (!_referenceIdToObjectMap.TryAdd(referenceId, value))
                    {
                        throw new JsonException();
                    }
                }

                public override string GetReference(object value, out bool alreadyExists)
                {
                    if (_objectToReferenceIdMap.TryGetValue(value, out string referenceId))
                    {
                        alreadyExists = true;
                    }
                    else
                    {
                        _referenceCount++;
                        referenceId = _referenceCount.ToString();
                        _objectToReferenceIdMap.Add(value, referenceId);
                        alreadyExists = false;
                    }

                    return referenceId;
                }

                public override object ResolveReference(string referenceId)
                {
                    if (!_referenceIdToObjectMap.TryGetValue(referenceId, out object value))
                    {
                        throw new JsonException();
                    }

                    return value;
                }
            }
        }
    }
}