404 error after adding Web API to an existing MVC Web Application

There's a great question here: https://stackoverflow.com/questions/11990036/how-to-add-web-api-to-an-existing-asp-net-mvc-4-web-application-project

Unfortunately, it wasn't enough to solve my problem. I've tried twice to be sure I haven't done anything wrong. I right clicked on "Controllers" and added the item "Web API 2 Controller with actions, using Entity Framework" where I selected my model class and db context. Everything went fine... but still... everytime I've tried to access /api/Rest I was getting a 404 error (The name of my Controller is RestController).

It's working!!! I didn't want to believe, but guess what, the problem was related with the Global.asax routing order.

While it doesn't work with:

protected void Application_Start()
{
    AreaRegistration.RegisterAllAreas();
    FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
    RouteConfig.RegisterRoutes(RouteTable.Routes);
    GlobalConfiguration.Configure(WebApiConfig.Register); //I AM THE 4th
    BundleConfig.RegisterBundles(BundleTable.Bundles);
}      

It works with:

protected void Application_Start()
{
    AreaRegistration.RegisterAllAreas();
    GlobalConfiguration.Configure(WebApiConfig.Register); //I AM THE 2nd
    FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
    RouteConfig.RegisterRoutes(RouteTable.Routes);
    BundleConfig.RegisterBundles(BundleTable.Bundles);
}      

Crazy, I know.

If you want to use WebAPI inside an existing MVC (5) project you have to do the following steps:
1.Add WebApi packages:

Microsoft.AspNet.WebApi
Microsoft.AspNet.WebApi.Client
Microsoft.AspNet.WebApi.Core
Microsoft.AspNet.WebApi.WebHost
Newtonsoft.Json

2.Add WebApiConfig.cs file to App_Start folder:

using System.Web.Http;

namespace WebApiTest
{
    public static class WebApiConfig
    {
        public static void Register(HttpConfiguration config)
        {
            // Web API configuration and services

            // Web API routes
            config.MapHttpAttributeRoutes();

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

3.Add the following line to Glabal.asax:

GlobalConfiguration.Configure(WebApiConfig.Register);

Important note: you have to add above line exactly after AreaRegistration.RegisterAllAreas();

protected void Application_Start()
{
    AreaRegistration.RegisterAllAreas();
    //\\
    GlobalConfiguration.Configure(WebApiConfig.Register);
    //\\
    FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
    RouteConfig.RegisterRoutes(RouteTable.Routes);
    BundleConfig.RegisterBundles(BundleTable.Bundles);
}

 

"When adding new routes ALWAYS KEEP IN MIND that you have to add specific route on the top followed by more generic route in the end. Otherwise, your web app will never receive proper routing."

The above is the citation from here: http://www.codeproject.com/Tips/771809/Understanding-the-Routing-Framework-in-ASP-NET-MVC

I know the answer is already given, but this could help to understand why we need to put GlobalConfiguration.Configure(WebApiConfig.Register); before RouteConfig.RegisterRoutes(RouteTable.Routes);