Configure input/output formatters on controllers with ASP.NET Core 2.1

Question

I am in the process of rewriting an old ASP.NET WebAPI 2.1 project to ASP.NET Core MVC 2.1. One of the problem I am facing is about porting the old behavior of the service which configure the input and output formatters through custom attributes whom implement IControllerConfiguration interface. I have not been able to find a replacement for this interface nor any alternative to configure formatters on controller-basis, other than injecting them at global level with the AddMvc(options) method.

Answer 1

I haven't found anything that can be configured at the controller level, but I did find a solution that involves changes to each action where you need this functionality. In my case I needed to customize the JSON serializer settings, which can be done like this for the output:

[HttpGet]
public IActionResult Get()
{
    ...
    return Json(result, _serializerSettings);
}

and like this for input:

[HttpPost]
public IActionResult Post([FromBodyCustomSerializationSettings]MyPostDto postDto)
{
    ...
}

class FromBodyCustomSerializationSettingsAttribute : ModelBinderAttribute
{
    public FromBodyCustomSerializationSettingsAttribute() : base(typeof(MyModelBinder))
    {
        BindingSource = BindingSource.Body;
    }
}

class MyModelBinder : IModelBinder
{
    private readonly BodyModelBinder _bodyModelBinder;

    public MyModelBinder(IHttpRequestStreamReaderFactory readerFactory, ILoggerFactory loggerFactory, IOptions<MvcOptions> options, IOptions<MvcJsonOptions> jsonOptions, ArrayPool<char> charPool, ObjectPoolProvider objectPoolProvider)
    {
        var formatters = options.Value.InputFormatters.ToList();
        int jsonFormatterIndex = formatters.FirstIndexOf(formatter => formatter is JsonInputFormatter);
        JsonSerializerSettings myCustomSettings = ...
        formatters[jsonFormatterIndex] = new JsonInputFormatter(loggerFactory.CreateLogger("MyCustomJsonFormatter"), myCustomSettings, charPool, objectPoolProvider, options.Value, jsonOptions.Value);
        _bodyModelBinder = new BodyModelBinder(formatters, readerFactory, loggerFactory, options.Value);
    }

    public Task BindModelAsync(ModelBindingContext bindingContext)
    {
        return _bodyModelBinder.BindModelAsync(bindingContext);
    }
}

Answer 2

Actually I found a way. I created an attribute which also implements IResultFilter and here is the OnResultExecuting method, where the magic happens:

public void OnResultExecuting(ResultExecutingContext context)
{
  var objectResult = context.Result as ObjectResult;
  if (objectResult != null)
  {
    var serializerSettings = new JsonSerializerSettings
    {
        ContractResolver = new DefaultContractResolver()
    };

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

    objectResult.Formatters.Add(jsonFormatter);
  }
}

Basically here I am injecting a custom JSON formatter in every object result, before it is sent to the client. It appears (but I did not find any documentation about this) that in this way ASP.NET Core MVC prefers the injected formatter over the globally defined one.

I hopes it helps other because I was struggling on this...

Answer 3

In order to be able to use @dcstraw's code in ASP .Net Core 3.1, I needed to modify MyModelBinder implementation slightly:

class MyModelBinder : IModelBinder
{
    private readonly BodyModelBinder _bodyModelBinder;

    public MyModelBinder(IHttpRequestStreamReaderFactory readerFactory, ILoggerFactory loggerFactory, IOptions<MvcOptions> options, IOptions<MvcNewtonsoftJsonOptions> jsonOptions, ArrayPool<char> charPool, ObjectPoolProvider objectPoolProvider)
    {
        var formatters = options.Value.InputFormatters.ToList();
        int jsonFormatterIndex = formatters.FindIndex(formatter => formatter is NewtonsoftJsonInputFormatter);
        JsonSerializerSettings myCustomSettings = ...
        formatters[jsonFormatterIndex] = new NewtonsoftJsonInputFormatter(loggerFactory.CreateLogger("MyCustomJsonFormatter"), myCustomSettings, charPool, objectPoolProvider, options.Value, jsonOptions.Value);
        _bodyModelBinder = new BodyModelBinder(formatters, readerFactory, loggerFactory, options.Value);
    }

    public Task BindModelAsync(ModelBindingContext bindingContext)
    {
        return _bodyModelBinder.BindModelAsync(bindingContext);
    }
}

Basically :

• JsonInputFormatter changes into NewtonsoftJsonInputFormatter
• MvcJsonOptions changes into MvcNewtonsoftJsonOptions