Why and How to implement Dependency Injection Pattern?

Benefits

By using dependency injection, you solve two main problems,

1- Unit Test
2- Instantiation

Unit Test

You can resolve this by implementing interface of every class that is required to instantiated in any method or implementation. Always pass interface as parameter in constructor rather than making variables and instantiate in the class. For unit test, you can send mocked (the dummy) instantiation of that interface to that class.

Before

        public class Order
        {
            public string ID { get; set; }
            public string Detail { get; set; }

            public void Process()
            {
            }
        }
        public class Cart
        {
            public void ProcessOrder()
            {
                Order order = new Order();
                order.Process();
            }
        }

How to add Web API to an existing ASP.NET MVC Web Application

The steps I needed to perform were:

1. Add reference to System.Web.Http.WebHost.
2. Add App_Start\WebApiConfig.cs (see code snippet below).
3. Import namespace System.Web.Http in Global.asax.cs.
4. Call WebApiConfig.Register(GlobalConfiguration.Configuration) in MvcApplication.Application_Start() (in file Global.asax.cs), before registering the default Web Application route as that would otherwise take precedence.
5. Add a controller deriving from System.Web.Http.ApiController.

I could then learn enough from the tutorial (Your First ASP.NET Web API) to define my API controller.

App_Start\WebApiConfig.cs:

using System.Web.Http;

class WebApiConfig
{
    public static void Register(HttpConfiguration configuration)
    {
        configuration.Routes.MapHttpRoute("API Default", "api/{controller}/{id}",
            new { id = RouteParameter.Optional });

// To return objects in Json format (Not XML Format),

var appXmlType = configuration.Formatters.XmlFormatter.SupportedMediaTypes.FirstOrDefault(t => t.MediaType == "application/xml");
            configuration.Formatters.XmlFormatter.SupportedMediaTypes.Remove(appXmlType);

    }
}

Global.asax.cs:

using System.Web.Http;

...

protected void Application_Start()
{
    AreaRegistration.RegisterAllAreas();

    RegisterGlobalFilters(GlobalFilters.Filters);
    WebApiConfig.Register(GlobalConfiguration.Configuration);
    RegisterRoutes(RouteTable.Routes);
    BundleConfig.RegisterBundles(BundleTable.Bundles);
}

Entity Framework Code First Approach

Code First approach does not actually Mean that you can not utilize existing database. Instead, it lets you deal with existing database or you can start with code and Entity Framework will create new database. Let us See both approaches,

Existing Database

Open your project in Visual Studio 2013 and click Add New Item -> ADO.NET Entity Model

Click on the Code First from database option. This wizard will lead you to create model classes.