In this post, I am applying Dependency Injection to the NerdDinner application using Ninject. The controllers of NerdDinner application have Dependency Injection enabled constructors. So we can apply Dependency Injection through constructor without change any existing code. A Dependency Injection framework injects the dependencies into a class when the dependencies are needed. Dependency Injection enables looser coupling between classes and their dependencies and provides better testability of an application and it removes the need for clients to know about their dependencies and how to create them. If you are not familiar with Dependency Injection and Inversion of Control (IoC), read Martin Fowler’s article Inversion of Control Containers and the Dependency Injection pattern.
The Open Source Project NerDinner is a great resource for learning ASP.NET MVC. A free eBook provides an end-to-end walkthrough of building NerdDinner.com application. The free eBook and the Open Source Nerddinner application are extremely useful if anyone is trying to lean ASP.NET MVC. The first release of Nerddinner was as a sample for the first chapter of Professional ASP.NET MVC 1.0. Currently the application is updating to ASP.NET MVC 2 and you can get the latest source from the source code tab of Nerddinner at http://nerddinner.codeplex.com/SourceControl/list/changesets. I have taken the latest ASP.NET MVC 2 source code of the application and applied Dependency Injection using Ninject and Ninject extension Ninject.Web.Mvc.
Ninject & Ninject.Web.Mvc
Ninject is available at http://github.com/ninject and Ninject.Web.Mvc is available at http://github.com/ninject/ninject.web.mvc
Ninject is a lightweight and a great dependency injection framework for .NET. Ninject is a great choice of dependency injection framework when building ASP.NET MVC applications. Ninject.Web.Mvc is an extension for ninject which providing integration with ASP.NET MVC.
Controller constructors and dependencies of NerdDinner application
Listing 1 – Constructor of DinnersController
Listing 2 – Constrcutor of AccountController
Listing 3 – Constructor of AccountMembership – Concrete class of IMembershipService
Dependencies of NerdDinner
DinnersController, RSVPController SearchController and ServicesController have a dependency with IDinnerRepositiry. The concrete implementation of IDinnerRepositiry is DinnerRepositiry. AccountController has dependencies with IFormsAuthentication and IMembershipService. The concrete implementation of IFormsAuthentication is FormsAuthenticationService and the concrete implementation of IMembershipService is AccountMembershipService. The AccountMembershipService has a dependency with ASP.NET Membership Provider.
Dependency Injection in NerdDinner using Ninject
The below steps will configure Ninject to apply controller injection in NerdDinner application.
Step 1 – Add reference for Ninject
Open the NerdDinner application and add reference to Ninject.dll and Ninject.Web.Mvc.dll. Both are available from http://github.com/ninject and http://github.com/ninject/ninject.web.mvc
Step 2 – Extend HttpApplication with NinjectHttpApplication
Ninject.Web.Mvc extension allows integration between the Ninject and ASP.NET MVC. For this, you have to extend your HttpApplication with NinjectHttpApplication. Open the Global.asax.cs and inherit your MVC application from NinjectHttpApplication instead of HttpApplication.
Then the Application_Start method should be replace with OnApplicationStarted method. Inside the OnApplicationStarted method, call the RegisterAllControllersIn() method.
The RegisterAllControllersIn method will enables to activating all controllers through Ninject in the assembly you have supplied .We are passing the current assembly as parameter for RegisterAllControllersIn() method. Now we can expose dependencies of controller constructors and properties to request injections
Step 3 – Create Ninject Modules
We can configure your dependency injection mapping information using Ninject Modules.Modules just need to implement the INinjectModule interface, but most should extend the NinjectModule class for simplicity.
The above Binding inforamtion specified in the Load method tells the Ninject container that, to inject instance of DinnerRepositiry when there is a request for IDinnerRepositiry and inject instance of FormsAuthenticationService when there is a request for IFormsAuthentication and inject instance of AccountMembershipService when there is a request for IMembershipService. The AccountMembershipService class has a dependency with ASP.NET Membership provider. So we configure that inject the instance of Membership Provider. When configuring the binding information, you can specify the object scope in you application.
There are four built-in scopes available in Ninject:
- Transient - A new instance of the type will be created each time one is requested. (This is the default scope). Binding method is .InTransientScope()
- Singleton - Only a single instance of the type will be created, and the same instance will be returned for each subsequent request. Binding method is .InSingletonScope()
- Thread - One instance of the type will be created per thread. Binding method is .InThreadScope()
- Request - One instance of the type will be created per web request, and will be destroyed when the request ends. Binding method is .InRequestScope()
Step 4 – Configure the Ninject Kernel
Once you create NinjectModule, you load them into a container called the kernel. To request an instance of a type from Ninject, you call the Get() extension method. We can configure the kernel, through the CreateKernel method in the Global.asax.cs.
Here we are loading the Ninject Module (ServiceModule class created in the step 3) onto the container called the kernel for performing dependency injection.
You can download the source code from http://nerddinneraddons.codeplex.com. I put the modified source code of NerdDinner app onto CodePlex repository. The repository will update with more add-ons for the NerdDinner application.