c# - CreatedAtAction Returns 500 on Success

Question

Overview
-Blazor Server project using REST API.
-HttpClient calls my API using PostJsonAsync<>
-Db is updated successfully and returns expected JSON
-API Controller Try/Catch does not throw exception, executes return CreatedAtAction()
-Output shows API returns 500: Internal Server Error
-When executing POST using Swagger, same line is executed and returns 201

I'm at a loss for what is going wrong. I've only been coding since July, so I'm hoping it's just my lack of experience that's blinding me to the obvious problem in my code. This is the offending method:

        public async Task<ActionResult<PlayerCharacter>> CreatePlayerCharacter(PlayerCharacter playerCharacter)
        {

            try
            {
                if (playerCharacter != null)
                {
                    var newPlayerCharacter = await _playerCharacterRepository.AddPlayerCharacter(playerCharacter);

                    //This is the last line of code to execute before the web app crashes with Status Code 500. Executes fine and returns 201 from Swagger.
                    return CreatedAtAction(nameof(GetPlayerCharacter), new { id = newPlayerCharacter.Id }, newPlayerCharacter);
                }

                return BadRequest();
            }
            catch (Exception e)
            {
                //This break point is not reached
                Console.WriteLine(e.StackTrace);
                return StatusCode(StatusCodes.Status500InternalServerError, "Error posting data to the database.");
            }
        }

GitHub Repo

question from:https://stackoverflow.com/questions/65873276/createdataction-returns-500-on-success

Answer 1

The root cause of the problem is as yet not fully understood, but I do know how I fixed it.

After poke taught me how to look at the ASP.NET Core logs, I discovered this exception:
System.Text.Json.JsonException: A possible object cycle was detected. This can either be due to a cycle or if the object depth is larger than the maximum allowed depth of 32. Consider using ReferenceHandler.Preserve on JsonSerializerOptions to support cycles.

Researching that lead me to several resources:

• Solution from 2017 which lead to...
  
• Solution from 2019 which links out to The future of JSON in .NET Core 3.0
  
• I also referenced the EntityFrameworkCore.com Knowledge Base

The solution may be that I should refactor my code to use DTOs (something I'll have to read up on because I don't understand that yet). In the meantime, the bandaid that will allow me to proceed is:

1. Install NuGet Package Microsoft.AspNetCore.Mvc.NewtonsoftJson
2. In Statup.cs in ConfigureServices add

services.AddControllers().AddNewtonsoftJson(options =>
                options.SerializerSettings.ReferenceLoopHandling = ReferenceLoopHandling.Ignore);

Thank you to everyone who helped me think through this. I greatly appreciate your assistance!