ActionResultHelper.cs

using System;
using System.Collections.Generic;
using System.Text;
using Microsoft.AspNetCore.Mvc;
using Newtonsoft.Json;

namespace Gnarlywood.Core.Sites
{
    public static class ActionResultHelper
    {
        public static IActionResult Json(object content, int? statusCode = null)
        {
            return new ContentResult() {Content = JsonConvert.SerializeObject(content), ContentType = "application/json", StatusCode = statusCode};
        }


        /// <summary>
        /// https://tools.ietf.org/html/rfc7807 problem response
        /// </summary>
        /// <param name="problem"></param>
        /// <param name="statusCode"></param>
        /// <returns></returns>
        public static IActionResult Problem(ProblemBase problem, int statusCode = 400)
        {
            return new ContentResult() { Content = JsonConvert.SerializeObject(problem), ContentType = "application/problem+json", StatusCode = statusCode };
        }

        public static IActionResult NotFoundProblem(string missingType, string instance = null)
        {
            return Problem(new ProblemBase()
                {Title = "Not Found", Type = missingType, Description = $"Object of Type {missingType} was not found", Instance = instance}, 404);
        }
    }
}

ProblemBase.cs

using System;
using Newtonsoft.Json;

namespace Gnarlywood.Core.Sites
{
    public class ProblemBase
    {
        [JsonProperty("title")]
        public string Title { get; set; }
        [JsonProperty("type")]
        public string Type { get; set; }
        [JsonProperty("description")]
        public string Description { get; set; }

        [JsonProperty("instance", DefaultValueHandling = DefaultValueHandling.Ignore)]
        public string Instance { get; set; }
    }
}