Cannot replace default JSON contract resolver in ASP.NET Core 3

Question

After creating basic Web API project based on .NET Core 3.0 framework, all API responses were coming in camel case. I installed SwashBuckle Swagger + built-in JSON serializer from System.Text.Json, specifically, to display enums as strings, everything worked as before. Then, I decided to switch to NSwag + NewtonSoftJson, because of some limitations of built-in serializer with dynamic and expando objects. Now, all API responses are displayed in PascalCase and I cannot change neither naming policy, nor even create custom contract resolver.

Example

https://forums.asp.net/t/2138758.aspx?Configure+SerializerSettings+ContractResolver

Question

I suspect that maybe some package overrides contract resolver behind the scene. How to make sure that API service uses ONLY custom contract resolver that I assign at startup and ignores all other similar settings?

Custom JSON contract resolver:

public class CustomContractResolver : IContractResolver
{
  private readonly IHttpContextAccessor _context;
  private readonly IContractResolver _contract;
  private readonly IContractResolver _camelCaseContract;

  public CustomContractResolver(IHttpContextAccessor context)
  {
    _context = context;
    _contract = new DefaultContractResolver();
    _camelCaseContract = new CamelCasePropertyNamesContractResolver();
  }

  // When API endpoint is hit, this method is NOT triggered

  public JsonContract ResolveContract(Type value)
  {
    return _camelCaseContract.ResolveContract(value);
  }
}

Controller:

[ApiController]
public class RecordsController : ControllerBase
{
  [HttpGet]
  [Route("services/records")]
  [ProducesResponseType(typeof(ResponseModel<RecordEntity>), 200)]
  public async Task<IActionResult> Records([FromQuery] QueryModel queryModel)
  {
    var response = new ResponseModel<RecordEntity>();

    return Content(JsonConvert.SerializeObject(response), "application/json"); // NewtonSoft serializer
  }
}

Startup.cs

public void ConfigureServices(IServiceCollection services)
{
  services
    .AddCors(o => o.AddDefaultPolicy(builder => builder
      .AllowAnyOrigin()
      .AllowAnyHeader()
      .AllowAnyMethod()));

  services
    .AddControllers(o => o.RespectBrowserAcceptHeader = true)
    /*
    .AddJsonOptions(o =>
    {
      o.JsonSerializerOptions.Converters.Add(new JsonStringEnumConverter());
      o.JsonSerializerOptions.DictionaryKeyPolicy = JsonNamingPolicy.CamelCase;
      o.JsonSerializerOptions.PropertyNamingPolicy = JsonNamingPolicy.CamelCase;
    })
    */
    .AddNewtonsoftJson(o =>
    {
      o.UseCamelCasing(true);
      o.SerializerSettings.Converters.Add(new StringEnumConverter());
      //o.SerializerSettings.ContractResolver = new CamelCasePropertyNamesContractResolver { NamingStrategy = new CamelCaseNamingStrategy() };
      o.SerializerSettings.ContractResolver = new CustomContractResolver(new HttpContextAccessor());
    });

  services.AddOpenApiDocument(o =>   // NSwag
  {
    o.PostProcess = document =>
    {
      document.Info.Version = "v1";
      document.Info.Title = "Demo API";
    };
  });

  DataConnection.DefaultSettings = new ConnectionManager(DatabaseOptionManager.Instance); // LINQ to DB
}

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
  if (env.IsDevelopment())
  {
    app.UseDeveloperExceptionPage();
  }

  app.UseCors(o => o.AllowAnyOrigin().AllowAnyMethod().AllowAnyHeader());
  app.UseRouting();
  app.UseAuthorization();
  app.UseEndpoints(o => o.MapControllers());

  app.UseOpenApi();                                // NSwag
  app.UseSwaggerUi3(o => o.Path = "/v2/docs");
  app.UseReDoc(o => o.Path = "/v1/docs");
}

Answer 1

Still don't understand why custom contract resolver is not triggered by API endpoint, but found a combination that works for me to switch API to camel case. Feel free to explain why it works this way.

services.AddControllers(o => o.RespectBrowserAcceptHeader = true)

  // Options for System.Text.Json don't affect anything, can be uncommented or removed

  //.AddJsonOptions(o =>
  //{
  //  o.JsonSerializerOptions.Converters.Add(new JsonStringEnumConverter());
  //  o.JsonSerializerOptions.DictionaryKeyPolicy = JsonNamingPolicy.CamelCase;
  //  o.JsonSerializerOptions.PropertyNamingPolicy = JsonNamingPolicy.CamelCase;
  //})

  .AddNewtonsoftJson(o =>
  {
    o.UseCamelCasing(true);
    o.SerializerSettings.Converters.Add(new StringEnumConverter());

    // This option below breaks global settings, so had to comment it

    //o.SerializerSettings.ContractResolver = new CamelCasePropertyNamesContractResolver 
    //{
    //  NamingStrategy = new CamelCaseNamingStrategy()
    //};
  });

JsonConvert.DefaultSettings = () => new JsonSerializerSettings
{
  ContractResolver = new CamelCasePropertyNamesContractResolver()
};

Idea was taken from this article.

NewtonSoft allows to set global serialization settings disregarding MVC, Web API, and other frameworks.