Difference between ApiController and Controller in ASP.NET MVC

Use Controller to render your normal views. ApiController action only return data that is serialized and sent to the client.

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Quote:

Note If you have worked with ASP.NET MVC, then you are already familiar with controllers. They work similarly in Web API, but controllers in Web API derive from the ApiController class instead of Controller class. The first major difference you will notice is that actions on Web API controllers do not return views, they return data.

ApiControllers are specialized in returning data. For example, they take care of transparently serializing the data into the format requested by the client. Also, they follow a different routing scheme by default (as in: mapping URLs to actions), providing a REST-ful API by convention.

You could probably do anything using a Controller instead of an ApiController with the some(?) manual coding. In the end, both controllers build upon the ASP.NET foundation. But having a REST-ful API is such a common requirement today that WebAPI was created to simplify the implementation of a such an API.

It's fairly simple to decide between the two: if you're writing an HTML based web/internet/intranet application - maybe with the occasional AJAX call returning json here and there - stick with MVC/Controller. If you want to provide a data driven/REST-ful interface to a system, go with WebAPI. You can combine both, of course, having an ApiController cater AJAX calls from an MVC page.

To give a real world example: I'm currently working with an ERP system that provides a REST-ful API to its entities. For this API, WebAPI would be a good candidate. At the same time, the ERP system provides a highly AJAX-ified web application that you can use to create queries for the REST-ful API. The web application itself could be implemented as an MVC application, making use of the WebAPI to fetch meta-data etc.

Which would you rather write and maintain?

ASP.NET MVC

public class TweetsController : Controller {
  // GET: /Tweets/
  [HttpGet]
  public ActionResult Index() {
    return Json(Twitter.GetTweets(), JsonRequestBehavior.AllowGet);
  }
}

ASP.NET Web API

public class TweetsController : ApiController {
  // GET: /Api/Tweets/
  public List<Tweet> Get() {
    return Twitter.GetTweets();
  }
}

I love the fact that ASP.NET Core's MVC6 merged the two patterns into one because I often need to support both worlds. While it's true that you can tweak any standard MVC Controller (and/or develop your own ActionResult classes) to act & behave just like an ApiController, it can be very hard to maintain and to test: on top of that, having Controllers methods returning ActionResult mixed with others returning raw/serialized/IHttpActionResult data can be very confusing from a developer perspective, expecially if you're not working alone and need to bring other developers to speed with that hybrid approach.

The best technique I've come so far to minimize that issue in ASP.NET non-Core web applications is to import (and properly configure) the Web API package into the MVC-based Web Application, so I can have the best of both worlds: Controllers for Views, ApiControllers for data.

In order to do that, you need to do the following:

• Install the following Web API packages using NuGet: Microsoft.AspNet.WebApi.Core and Microsoft.AspNet.WebApi.WebHost.
• Add one or more ApiControllers to your /Controllers/ folder.
• Add the following WebApiConfig.cs file to your /App_Config/ folder:

using System.Web.Http;

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        // Web API routes
        config.MapHttpAttributeRoutes();

        config.Routes.MapHttpRoute(
            name: "DefaultApi",
            routeTemplate: "api/{controller}/{id}",
            defaults: new { id = RouteParameter.Optional }
        );
    }
}

Finally, you'll need to register the above class to your Startup class (either Startup.cs or Global.asax.cs, depending if you're using OWIN Startup template or not).

Startup.cs

 public void Configuration(IAppBuilder app)
 {
    // Register Web API routing support before anything else
    GlobalConfiguration.Configure(WebApiConfig.Register);

    // The rest of your file goes there
    // ...
    AreaRegistration.RegisterAllAreas();
    FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
    RouteConfig.RegisterRoutes(RouteTable.Routes);
    BundleConfig.RegisterBundles(BundleTable.Bundles);

    ConfigureAuth(app);
    // ...
}

Global.asax.cs

protected void Application_Start()
{
    // Register Web API routing support before anything else
    GlobalConfiguration.Configure(WebApiConfig.Register);

    // The rest of your file goes there
    // ...
    AreaRegistration.RegisterAllAreas();
    FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
    RouteConfig.RegisterRoutes(RouteTable.Routes);
    BundleConfig.RegisterBundles(BundleTable.Bundles);
    // ...
}

This approach - together with its pros and cons - is further explained in this post I wrote on my blog.

Quick n Short Answer

If you want to return a view, you should be in "Controller".

Normal Controller - ASP.NET MVC: you deal with normal "Controller" if you are in ASP.net Web Application. You can create Controller-Actions and you can return Views().

ApiController Controller: you create ApiControllers when you are developing ASP.net REST APIs. you can't return Views (though you can return Json/Data for HTML as string). These apis are considered as backend and their functions are called to return data not the view

Please dont forgt to mark this as answer, take care

More Details here

Every method in Web API will return data (JSON) without serialization.

However, in order to return JSON Data in MVC controllers, we will set the returned Action Result type to JsonResult and call the Json method on our object to ensure it is packaged in JSON.