Red: Dependency Injection in ASP.NET Web API 2

Step 1: - Open Visual Studio 2015 => Go to File Menu => New => Project...

Step 2: - In the Installed Templates list, select Visual C# => Web

Step 3: - Select ASP.Net Web Application (.NET Framework) from the Web list => Type WebApi in the Name box => Click OK

Step 4: - Select Empty template from ASP.NET Templates List and Checked Web API check box => Click OK

Step 5: - Project Structure

Step 6: - Right Click on Project Root folder in Solution Explorer => Add => Click on New Folder => Type folder name Repository

Step 7: - Right Click on Repository folder in Solution Explorer => Add => Click New Items... => Expand Visual C# from left pane => Select Code =>Select Interface from middle pane => Type ITest.cs in the Name box => Click Add

Step 8: - Add following code in ITest.cs file

namespace WebApi.Repository
{
public interface ITest
{
string GetMessage();
}

}

Step 9: - Right Click on Repository folder in Solution Explorer => Add => Click New Items... => Expand Visual C# from left pane => Select Code =>Select Class from middle pane => Type Test.cs in the Name box => Click Add

Step 10: - Add following code in Test.cs file

Note:- Here Test.cs implement the ITest.cs interface

namespace WebApi.Repository
{
  public class Test : ITest
{
  public string GetMessage()
{
  return "Welcome to Web Api.";
}
}

}

Step 11: - Right Click on Controllers folder => Add => Controller... => Select MVC 5 Controller - Empty => Click Add => Type HomeController in Controller name box => Click Add

Step 12: - Add following code in HomeController file

Note:- Here we have create parameter-less HomeController constructor to initialize the object of Test class to call the GetMessage().

using System.Web.Http;
using WebApi.Repository;

namespace WebApi.Controllers
{
  public class HomeController : ApiController
{
  private readonly ITest _test;
  public HomeController()
{
_test = new Test();
}
[HttpGet]
  public string Index()
{
  return _test.GetMessage();
}
}

}

We have done the simple Web Api project without using Dependency injection(DI).

Launch the project => Enter URL in browser.

http://localhost:51896/api/home/index

Note:- Port defer from system to system. Check your Port.

Till here our project work as expected. Now implement the Dependency injection(DI) in it.

Step 13: - Open HomeController file and change HomeController constructor and inject the service.

using System.Web.Http;
using WebApi.Repository;

namespace WebApi.Controllers
{
  public class HomeController : ApiController
{
  private readonly ITest _test;
  public HomeController(ITest test)
{
_test = test;
}
[HttpGet]
  public string Index()
{
  return _test.GetMessage();
}
}

}

Launch the project => Enter URL in browser.

http://localhost:51896/api/home/index

Note:- This time we will received an error “An error occurred when trying to create a controller of type 'HomeController'. Make sure that the controller has a parameterless public constructor.”

This error indicates that Web Api by default not support the parameterized constructor and DI. For that we need to use one of Unity library to fix this issue.

Step 14: - Open Solution Explorer => Right Click on References => Click on Manage NuGet Packages… => Search Unity.AspNet.WebApi => Choose Unity.AspNet.WebApi from list => Click to Install

Step 15: - Open Solution Explorer => Expand App_Start folder

Note:- Here 2 file added in when we install the Unity.AspNet.WebApi DLL that is UnityWebApiActivator.cs and UnityConfig.cs

Step 16: - Open UnityConfig.cs => Go to RegisterTypes function => Do the following changes

Launch the project => Enter URL in browser.

http://localhost:51896/api/home/index

Here we have successfully implemented the Dependency injection(DI) in our Web Api project.

To read setting from web.config file we need to add one more package Unity.Configuration in our solution.

Step 17 : - Open Solution Explorer => Right Click on References => Click on Manage NuGet Packages… => Search Unity.Configuration => Choose Unity.Configuration from list => Click to Install