C#/ASP.NET - Calling Web API operation via URL? : HTTP Error 500

I'm developing an ASP.NET MVC 4 application. Getting information from the database and displaying it on the webpage went well at this stage. Then I decided I'd like to develop a Web API as well side-by-side with my application's progress. So far so good, until I tried to test it using a URL on the local host.

When I tried it out, I got an HTTP Error 500.

I ran the application from VS 2010 and it opened up http://localhost:23375/ in my browser. At the end of this, I appended my API call, bringing it to:

http://localhost:23375/api/Performance/ShowMachines

When I hit enter, I get the error. Why is this so and how can I resolve it?

Here is the relevant code:

PerformanceController.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Web.Http;

using PerfMon.Models;

namespace PerfMon.Controllers
{
    public class PerformanceController : ApiController
    {

        PerfMonDataRepository perfRep = new PerfMonDataRepository();

        // GET /performance/machines
        [HttpGet]
        public IQueryable<Machine> ShowMachines()
        {
            return perfRep.GetAllMachines();
        }

    }
}

Global.asax.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Http;
using System.Web.Mvc;
using System.Web.Optimization;
using System.Web.Routing;

namespace PerfMon
{
    // Note: For instructions on enabling IIS6 or IIS7 classic mode, 
    // visit http://go.microsoft.com/?LinkId=9394801

public class WebApiApplication : System.Web.HttpApplication
{
    protected void Application_Start()
    {
        AreaRegistration.RegisterAllAreas();

        FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
        RouteConfig.RegisterRoutes(RouteTable.Routes);
        BundleConfig.RegisterBundles(BundleTable.Bundles);
    }

    public static void RegisterRoutes(RouteCollection routes)
    {

        routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

        routes.MapHttpRoute(
            name: "Machines",
            routeTemplate: "api/{controller}/{action}",
            defaults: new { 
                controller = "Performance",
                action = "ShowMachines"                    
            }
        );
    }



}
}

Copyright Notice：Content Author:「praetor」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/11751201/c-asp-net-calling-web-api-operation-via-url-http-error-500