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API Design in ASP.NET Core Part III

This week I had the honor to give a training to some of the newly started young professionals in our organisation. The topic of the training was API design in ASP.NET Core. During this training we discussed multiple topics and a lot of interesting questions were raised. I'll try to tackle some of them with a blog post.

The question I try to tackle today is...

What does the [APIController] attribute do?

If you created your first Web API in ASP.NET Core (through dotnet new webapi -o MyFirstAPI), you ended up with a WeatherForecastController containing the following code:

On top of this controller class, you see above the Route attribute, the ApiController attribute. Adding this attribute on top of your controller has a bigger impact than you would think and enables the following behaviours:

  • Attribute routing requirement
  • Automatic HTTP 400 responses
  • Binding source parameter inference
  • Multipart/form-data request inference
  • Problem details for error status codes

Attribute routing requirement

The moment you start using the ApiController attribute, attribute routing becomes a requirement. This means that actions in this controller are no longer accessible via routes defined through UseEndpoints() or UseMVC().

Automatic HTTP 400 responses

Adding the ApiController attribute will automatically check the model state and return HTTP 400 responses when your model state has validation errors. This makes the ModelState.IsValid check obsolete in your action methods:

Behind the scenes, a ModelStateInvalidFilter is registered, which runs during OnActionExecuting event and executes the code above.

Binding source parameter inference

Normally depending on where the data is coming from (URL, headers, request body, …) you need to explicitly annotate the parameters of your action method with a binding attribute like [FromBody] or [FromQuery].

After adding the ApiController attribute, binding source parameters are inferred using the following rules:

Binding Attribute Inference Rule
[FromBody] Inferred for any complex type
[FromForm] Inferred for any parameters of type IFormFile or IFormFileCollection
[FromQuery] Inferred for all the other action parameters
[FromRoute] Inferred for any action parameter whose name matches a parameter in the route template
[FromHeader] No inferred rule

Multipart/form-data request inference

The [ApiController] attribute applies an inference rule for action parameters of type IFormFile and IFormFileCollection. The multipart/form-data request content type is inferred for these types.

Problem details for error status codes

Furthermore, ApiController introduces the ValidationProblem format for HTTP 400 responses. It is a machine-readable RFC 7807 compliant format:

More information: https://learn.microsoft.com/en-us/aspnet/core/web-api/?view=aspnetcore-6.0#apicontroller-attribute

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