AspNet core web Api usage of ApiControllerAttribute

Question

When I create a new controller in the API project, it generates a controller class with [ApiController] attribute, like this:

[ApiController]
public class TestController : ControllerBase
{
 //implementation
}

I've seen a few webapi projects where usage of this attribute is omitted. Microsoft documentation here says:

Indicates that a type and all derived types are used to serve HTTP API responses. The presence of this attribute can be used to target conventions, filters and other behaviors based on the purpose of the controller.

But still, I don't get the idea. Can someone explain what is the purpose of this attribute with the real-life example?

Check out [this](https://stackoverflow.com/a/16305878/9456781) for a real-life example and the accepted answer for further insight. — Tobias Tengler, Dec 02 '18 at 11:27
Always check the [documentation](https://learn.microsoft.com/en-us/aspnet/core/web-api/index?view=aspnetcore-2.1#annotation-with-apicontroller-attribute), its all pretty well documented. It just adds additional conventions for WebApi controllers, which makes sure you have to use less attributes to describe it (FromBody attribute becomes optional, model validation happens automatically etc) — Tseng, Dec 02 '18 at 13:36

score 35 · Accepted Answer · edited Jan 05 '23 at 17:40

This is well-explained in the docs: Annotation with ApiController attribute, which explains that adding the [ApiController] attribute to a controller does the following:

Automatic HTTP 400 responses

Adds an action filter that returns a 400 response if ModelState.IsValid is false.
Binding source parameter inference

Changes model-binding conventions. For example, [FromBody] is inferred for complex-type parameters.
Multipart/form-data request inference

Infers a Content-Type of multipart/form-data for parameters marked with [FromForm].
Attribute routing requirement

Mandates that all actions must be attribute-routed.

You can see how this is actually done in the source:

foreach (var actionModel in controllerModel.Actions)
{
    if (!isApiController && !actionModel.Attributes.OfType<IApiBehaviorMetadata>().Any())
    {
        continue;
    }

    EnsureActionIsAttributeRouted(controllerHasSelectorModel, actionModel);
    AddInvalidModelStateFilter(actionModel);
    InferParameterBindingSources(actionModel);
    InferParameterModelPrefixes(actionModel);
    AddMultipartFormDataConsumesAttribute(actionModel);
}

If you're not interested in any of the features described above, you can omit the attribute. It's also possible to suppress individual features by configuring the ApiBehaviorOptions class.

AspNet core web Api usage of ApiControllerAttribute

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