Use Custom Json Serialization Setting per Controller in ASP.NET Core 3.0+

CustomResultFilterAttribute.cs

using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Filters;
using Microsoft.AspNetCore.Mvc.Formatters;
using Newtonsoft.Json;
using Newtonsoft.Json.Serialization;
using System;
using System.Buffers;

namespace MyProject.Filters
{
    //Resources:
    //https://stackoverflow.com/questions/51769415/configure-input-output-formatters-on-controllers-with-asp-net-core-2-1
    //https://stackoverflow.com/questions/59634633/alternatives-to-jsonoutputformatter-in-asp-net-core-3-1-at-controller-level
    //Note Filters are only called on Controller class not ControllerBase class
    public class CustomResultFilterAttribute : Attribute, IResultFilter
    {
        public void OnResultExecuted(ResultExecutedContext context)
        {
        }

        public void OnResultExecuting(ResultExecutingContext context)
        {
            if (context.Result is ObjectResult objectResult)
            {
                var serializerSettings = new JsonSerializerSettings
                {
                    ContractResolver = new DefaultContractResolver(),
                    ReferenceLoopHandling = ReferenceLoopHandling.Ignore
                };

                var mvoptions = new MvcOptions();

                var jsonFormatter = new NewtonsoftJsonOutputFormatter(
                    serializerSettings,
                    ArrayPool<char>.Shared, mvoptions);

                objectResult.Formatters.Add(jsonFormatter);
            }
        }
    }
}

gistfile1.txt

using MyProject.Filters;
using Microsoft.AspNetCore.Mvc;

namespace MyProject.Controllers
{
    [ApiController]
    [CustomResultFilter]
    public class CustomController : Controller
    {
    }
}