Dependency Injection in .NET MVC 5 and WebApi

Intro

Dependency injection is a very popular concept today, usually as part of an MVC web application. Many new frameworks use DI by default, such as AngularJS and .NET Core, but what do those of us working on previous version of .NET MVC do? Well, there’s a solution.

MVC 5 provides many areas where we can inject our own behavior. This is part of the inversion of control philosophy that underlies many libraries. This means that to dependency inject our own services, all we need to do is create our own MVC controller factory and API controller factory, and come up with a method of registering our services.

This article will lead you through that process, along with full code examples.

Getting Started

Solution Structure

The structure should be as follows. Each item should be a project of the same name. The YourApp.Web project should be a .NET MVC web project, and all projects should output a DLL with the same name as the project. The remaining projects should be class libraries. Service should reference Interface, Web should reference Service and Windsor.

  • YourApp.Interface
  • YourApp.Service
  • YourApp.Web
  • YourApp.Windsor

Packages

The first thing you will need is to download a few packages. We will be using Castle.Windsor as our DI container library. Find this package on NuGet and add it to your project.

Castle Windsor NuGet
Castle Windsor NuGet

Structure

In order for DI to be effective, you will need a services “layer” that contains your business logic. This layer should be made of classes which handle your business logic. For the purpose of this example, we will use the interface and implementation below for a single injected service.

This file should go in YourApp.Interface.

namespace YourApp.Interface {
    public interface IDemoService {
        string SayHello();
    }
}

This file should go in YourApp.Service.

namespace YourApp.Service {
    public class DemoService : IDemoService {
        public string SayHello(){
            return "Hello";
        }
    }
}

Web Config

You will need to add this section to your web config.

<section name="castle" type="Castle.Windsor.Configuration.AppDomain.CastleSectionHandler, Castle.Windsor" />

You will need to add this information under the castle section. Add one component tag for each service you will be using. You can set the lifestyle as transient for now.

<castle>
    <components>
      <component id="Demo" service="YourApp.Service.IDemoService, YourApp.Interface" type="YourApp.Service.DemoService, YourApp.Service" lifestyle="transient"></component>
    </components>
</castle>

Defining Dependencies

In order for DI to work, we must first define what our dependencies are and what they resolve to. The installer below has been created for this purpose.

Windsor “installers” register the interfaces and their implementations. In this case, we are registering all controllers, all API controllers, and anything in our app configuration file (services above).

The configuration for the IActionInvoker is a little more special. What we’re doing here is specifying our own custom implementation, as well as passing a parameter to the constructor (the parameter is the DI container). You’ll see why we have to do this later.

There are two methods. One registers for the current assembly, which we won’t be using. The other registers using a specified assembly, which we will pass from our web project. The second method is the one you should be concerned with. The first is just a default which implements the necessary interface.

using Castle.MicroKernel.Registration;
using Castle.MicroKernel.SubSystems.Configuration;
using Castle.Windsor;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Web;
using System.Web.Http.Controllers;
using System.Web.Mvc;

namespace YourApp.Windsor {
    public class ServiceInstaller : IWindsorInstaller {
        public void Install(IWindsorContainer container, IConfigurationStore store) {
            container.Register(Classes.FromThisAssembly()
                   .BasedOn<IController>().LifestyleTransient());
            container.Register(Classes.FromThisAssembly()
                   .BasedOn<IHttpController>().LifestyleTransient());
            container.Register(Classes.FromThisAssembly()
                   .BasedOn<FilterAttribute>().LifestyleTransient());
            container.Register(Component.For<IActionInvoker>()
                   .ImplementedBy<WindsorActionInvoker>()
                   .DependsOn(
                      Dependency.OnValue("container", container)
                   ).LifestyleTransient());
            container.Install(Castle.Windsor.Installer.Configuration.FromAppConfig());
        }

        public void InstallFromAssembly(
           IWindsorContainer container, 
           IConfigurationStore store, 
           Assembly assembly
        ) {
            container.Register(Classes.FromAssembly(assembly)
                   .BasedOn<IController>().LifestyleTransient());
            container.Register(Classes.FromAssembly(assembly)
                   .BasedOn<IHttpController>().LifestyleTransient());
            container.Register(Classes.FromAssembly(assembly)
                   .BasedOn<FilterAttribute>().LifestyleTransient());
            container.Register(Component.For<IActionInvoker>()
                   .ImplementedBy<WindsorActionInvoker>()
                   .DependsOn(
                      Dependency.OnValue("container", container)
                   ).LifestyleTransient());
            container.Install(Castle.Windsor.Installer.Configuration.FromAppConfig());
        }
    }
}

Of course, this is just the base class. We’ll inherit from this to pass in our web assembly during installation.

The class below is the installer which will actually be called when the Windsor container is initialized.

using Castle.MicroKernel.Registration;
using Castle.MicroKernel.SubSystems.Configuration;
using Castle.Windsor;
using YourApp.Interface;
using YourApp.Windsor;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Web;
using System.Web.Http.Controllers;
using System.Web.Mvc;

namespace YourApp.Web.Windsor {
    public class WebServiceInstaller : ServiceInstaller, IWindsorInstaller {
        new public void Install(IWindsorContainer container, IConfigurationStore store) {
            InstallFromAssembly(container, store, Assembly.GetExecutingAssembly());
        }
    }
}

Now we just need to call our installer from the application start method in global.asax.cs.

protected void Application_Start() {
    // install windsor (find our class which implements IWindsorInstaller, and calls the Install method)
    Container = new WindsorContainer();
    Container.Install(FromAssembly.This());
}

Replacing the Controller Factory

ASP.NET MVC uses a controller factory to create instances of each controller when the application receives a request. This is what we will be replacing. Our new factory will resolve the controller type using our DI container, which will inject the services into the constructor.

Creating the New Factory

The new factory is pretty simple. When a controller is done being used, we release it from our container. When a controller is created, we resolve the dependencies through our container, and continue on as usual, calling the default factory methods.

using Castle.MicroKernel;
using Castle.Windsor;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.Web.Routing;

namespace YourApp.Windsor {
    public class WindsorControllerFactory : DefaultControllerFactory {
        private readonly IWindsorContainer _Container;

        public WindsorControllerFactory(IWindsorContainer container) {
            _Container = container;
        }

        public override void ReleaseController(IController controller) {
            _Container.Release(controller);  // The important part: release the component
        }

        protected override IController GetControllerInstance(
            RequestContext requestContext, 
            Type controllerType
        ) {
            if (controllerType == null) {
                throw new HttpException(404, 
                  string.Format(
                      "The controller for path '{0}' could not be found.", 
                      requestContext.HttpContext.Request.Path
                ));
            }

            Controller controller = (Controller)_Container.Resolve(controllerType);

            // new code
            if (controller != null) {
                // Don't worry about this yet. This will help us inject
                //dependencies into our action filters later
                controller.ActionInvoker = _Container.Resolve<IActionInvoker>();
            }

            return controller;
        }
    }
}

The above code follows the same process as the default controller factory that .NET provides, except that it uses the DI container to resolve the controller. The Resolve method is what injects our dependencies into the constructor.

Modify the Application Start

Now that we have this new controller factory, how will we use it? .NET MVC provides a point of extensibility for this, so we will add it there. Here is the code for this.

protected void Application_Start() {
    // install windsor
    Container = new WindsorContainer();
    Container.Install(FromAssembly.This());
    // use new controller factory
    ControllerBuilder.Current.SetControllerFactory(new WindsorControllerFactory(Container));
}

Replacing the Action Invoker

The action invoker is the class which determines how actions are called on a controller. This is the point of extensibility where we will dependency inject properties into the filters.

There is one issue however: by this point in the request lifecycle the action filter objects have already been created, so we can’t use the DI container Resolve method to inject the properties into the constructor.

To solve this problem, we will use the extension method below, which injects the services into an action filter for every matching public property.

using Castle.MicroKernel;
using Castle.MicroKernel.ComponentActivator;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Web;

namespace YourApp.Windsor {
    public static class WindsorExtension {
        public static void InjectProperties(this IKernel kernel, object target) {
            var type = target.GetType();
            var props = type.GetProperties(BindingFlags.Public | BindingFlags.Instance);
            foreach (var property in props) {
                if (property.CanWrite && kernel.HasComponent(property.PropertyType)) {
                    var value = kernel.Resolve(property.PropertyType);
                    try {
                        property.SetValue(target, value, null);
                    } catch (Exception ex) {
                        var message = string.Format(
                            @"Error setting property {0} on type {1}.
                            See inner exception for more information.", 
                            property.Name, type.FullName
                        );
                        throw new ComponentActivatorException(message, ex, null);
                    }
                }
            }
        }
    }
}

Creating the New Action Invoker

Now we can use this new extension method to loop through the already created action filters and inject their properties, continuing with the default invoke call after we’ve finished injecting our services.

using Castle.Windsor;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;

namespace YourApp.Windsor {
    public class WindsorActionInvoker : ControllerActionInvoker {
        readonly IWindsorContainer container;

        public WindsorActionInvoker(IWindsorContainer container) {
            this.container = container;
        }

        protected override ActionExecutedContext InvokeActionMethodWithFilters(
                ControllerContext controllerContext,
                IList filters,
                ActionDescriptor actionDescriptor,
                IDictionary<string, object> parameters) {
            foreach (IActionFilter actionFilter in filters) {
                container.Kernel.InjectProperties(actionFilter);
            }
            return base.InvokeActionMethodWithFilters(
                controllerContext, 
                filters, 
                actionDescriptor, 
                parameters
            );
        }

        protected override AuthorizationContext InvokeAuthorizationFilters(
           ControllerContext controllerContext, 
           IList filters, 
           ActionDescriptor actionDescriptor
        ) {
            foreach (IAuthorizationFilter authFilter in filters) {
                container.Kernel.InjectProperties(authFilter);
            }
            return base.InvokeAuthorizationFilters(
                controllerContext, 
                filters, 
                actionDescriptor
            );
        }
    }
}

Using the New Action Invoker

The controller factory has already been set up to use the new action invoker. That’s what the mysterious line was near the end of our controller factory. To recap, that was this:
controller.ActionInvoker = _Container.Resolve<IActionInvoker>();.
The _Container.Resolve call correctly calls our custom action invoker because we registered it in our installer at the beginning of the tutorial.

Replacing the WebApi Dependency Resolver

The .NET WebApi does not use the same concept of a controller factory (or at least all attempts to use it for DI have been unsuccessful). Instead, we will replace the default dependency resolver with our own.

Creating the New Dependency Resolver

Given that the dependency resolver for .NET WebApi is very similar to what any other DI container does, it is very easy to replace this with our own, using our Windsor DI container instead.

using Castle.MicroKernel;
using Castle.Windsor;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Http.Dependencies;

namespace YourApp.Windsor {
    public class WindsorDependencyResolver : System.Web.Http.Dependencies.IDependencyResolver {
        private readonly IWindsorContainer _container;

        public WindsorDependencyResolver(IWindsorContainer container) {
            if (container == null) {
                throw new ArgumentNullException("container");
            }

            _container = container;
        }
        public object GetService(Type t) {
            return _container.Kernel.HasComponent(t) ? _container.Resolve(t) : null;
        }

        public IEnumerable<object> GetServices(Type t) {
            return _container.ResolveAll(t).Cast<object>().ToArray();
        }

        public IDependencyScope BeginScope() {
            return new WindsorDependencyScope(_container);
        }

        public void Dispose() {

        }
    }
}

Updating the Application Start

To add our new resolver to the WebApi, we again find ourselves in the application start. here we can set our new resolver.

protected void Application_Start() {
    // install windsor
    Container = new WindsorContainer();
    Container.Install(FromAssembly.This());
    // resolve references for API controllers
    // adding a collection sub-resolver resolves things like List when you've only mapped Type.
    // thismay not be needed, but you should test your code with and without it to be sure
    Container.Kernel.Resolver.AddSubResolver(new CollectionResolver(Container.Kernel, true));
    // replace actual dependency resolver with our own
    var dependencyResolver = new WindsorDependencyResolver(Container);
    GlobalConfiguration.Configuration.DependencyResolver = dependencyResolver;
    // use new controller factory
    ControllerBuilder.Current.SetControllerFactory(new WindsorControllerFactory(Container));
}

Update the Application Code

Our solution is now ready for dependency injection, but our controllers and actions do not yet have properties to dependency inject into. We will show the updated structure of our controller and action filter classes.

Updating Controllers

Adding DI to our controllers is pretty easy. Our new controller factory will look for a constructor to inject the services into, matching any types with the services we specified. In this case, that’s our demo service. Our new controller constructor will look like this.

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Reflection;
using System.Web;
using System.Web.Mvc;
using System.Web.Security;

using YourApp.Interface;

namespace YourApp.Web.Controllers {
    public class HomeController : BaseController {
        private readonly IDemoService _DemoService;

        public HomeController() {

        }

        public HomeController(IDemoService demoService) {
            _DemoService = demoService;
        }

        //... actions here
    }
}

You can now reference _DemoService in any of your action methods. You can also easily replace the implementation, or add new services to be injected into the constructor.

Updating Action Filters

If you remember, our action filters have already been instantiated by the time we can inject the properties, so we had to create an extension method to do this. This problem also rules out using constructor injection, so we will take a different approach: property injection. Here is how that looks for a simple filter.

using YourApp.Interface;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Web.Mvc;

namespace YourApp.Web
{
    public class LayoutFilterAttribute : ActionFilterAttribute {
        // this property is filled in with our service by our special IActionInvoker
        public IDemoService _DemoService { get; set; }

        public LayoutFilterAttribute()
        {
            
        }

        public override void OnActionExecuted(ActionExecutedContext filterContext)
        {
            // inject data into layout
            filterContext.Controller.ViewBag.message = _DemoService.SayHello();
        }
    }
}

Conclusion

So that’s all there is to it! It can be quite a lengthy process, but it’s well worth the results, especially if upgrading to .NET Core is not an option, but you’d like to have fine control over your DI process.

The code given here is highly re-usable. If done correctly, you should have a YourApp.Windsor project that you can use in all .NET MVC 5 or WebApi projects for DI, which you can build and use as a dependency for any number of projects without modification.

DI will definitely help you more loosely couple the components of your application, as well as making changes in your application code much easier. You can also dependency inject services into your services, data access objects into your data access layer, and so on, making the pattern highly beneficial, especially when an application may use several implementations of the same service, or may be moved to another data store in the future. DI is good programming practice, and moreover it makes code much more pleasant to work in, and much easier to understand.

Another good takeaway is .NET’s approach to frameworks. They use inversion of control heavily, so there is always a point of extensibility that you can override to provide your own custom functionality. If you ever find yourself wishing a .NET library worked differently, remember: Microsoft has probably left the library heavily open to extension, so there’s no need to force it to do your bidding.

That’s all. I hope this becomes very useful in many people’s projects, it definitely has been for me.

Representational State Transfer (REST)

What is REST?

REST is an architecture which describes a system that transfers generally non-static content between a client and server. This content is called a resource, and is always some uniquely identifiable “thing”. RESTful services implemented on top of HTTP are a popular solution for web applications today.

Representational

REST is representational in the sense that every request must uniquely identify a resource. A resource is something which is uniquely identifiable. The meaning of uniquely identifiable can essentially be defined by the system, and is dependent on the level of granularity at which the system works.

For instance, on one system, perhaps a hospital is a unique resource, but on another system, each of the hospital’s buildings are considered independently, so you could not for instance request the completion date of the hospital, but only of a specific hospital building.

State

A RESTful service always returns stateful data. That is, it is returning the current or specified state of the specific resource, which is not necessarily and not usually composed of static data.

For instance, suppose that our hospital added a new wing. The resource representing the hospital, if it is a correct stateful representation, would then reflect this new building. Any request made prior to the addition of the new wing would return it’s current state – without the new wing.

This should not be confused with the statelessness of the requests. A RESTful server maintains no data about the state of the client or its requests.

Transfer

Transfer of course refers to the movement of data between a client and server. Data can flow both ways in a RESTful service, which usually supports the CRUD operations in addition to request types like HTTP OPTION, etc.

How did REST come about?

First, HTTP

The Hypertext Transfer Protocol, or HTTP, was the necessary precursor to what we consider a modern implementation of a RESTful service. HTTP provides a client-server architecture that focuses on text-based requests of documents. Because the text-based requests use URIs (unique resource identifiers), they are uniquely suited for use in a REST implementation, which is based on the concept of resources.

HTTP was originally proposed by Tim Berners-Lee, as a document storage and retrieval system between remote clients and servers. The original HTTP had only one method, GET, meaning that it would not be as suitable for a REST implementation as it is today.

HTTP soon added many methods, which made HTTP suited for a REST implementation. These methods included POST, PUT, and DELETE, which are used in today’s RESTful services to represent update, create, and delete operations respectively.

The Concept

The concept was coined in 2000 for a PhD dissertation by Roy Fielding. The concepts behind REST were used as the backbone for the URI standard used in HTTP requests. HTTP was therefore RESTful in its initial implementation (v 1.1). The difference between this and modern REST concepts is that the resource can be many more things than simply a static HTML document.

From this point, RESTful concepts were heavily adopted in the Web 2.0 age of asynchronous requests which loaded content in real-time into the browser. RESTful concepts enabled relatively simple and very consistent APIs to be created which abstracted this process heavily and eased implementation of complex applications handling asynchronous data requests.

What makes something RESTful?

There are five aspects necessary for a system to be considered RESTful, and one optional.

Client-Server Interactions

The architecture must start with a client-server model, where a single server hosts the unique resource, which a client may request.

Stateless Requests

This constraint means that the server cannot store session data from a client. Each request must include the session data necessary to execute the request.

Cacheability

Requests much be created in such a manner as to be identifiably cacheable or not. This allows an intermediate component to perform caching, without special system knowledge.

Layered Architecture

Each point of processing should not have awareness of other parts of the processing chain.

Uniform Interface

The system must define a consistent API, which decouples the requests from the implementation.

Transfer of Logic

An additional concept sometimes considered is the ability to transfer logic representations that can be executed on the client. This includes scripts, applets, etc. Many people are surprised to learn that this idea is part of the original PhD dissertation, and that implementation did not catch up to the possibility of the concepts for about a decade.

What is the purpose of REST?

The purpose of REST is to provide an architecture which creates sufficient abstraction in a large complex, distributed system of unique resources, so that a client-server model of resource access, alteration, and creation, can occur without significant complexity and overhead, even in a system of global scale such as the World Wide Web has become today.

How is REST Used?

REST today is the backbone of HTTP. All common HTTP requests are stateless, and conform to all the five criteria for a REST implementation.
However, the more common modern usage of REST today is in the implementation of a RESTful service on top of HTTP. These services are used to provide a layer of abstraction from data access and representation, so that client code can easily manipulate the resulting structures and compose requests to interact with this data.

Many times, the RESTful service is implemented as an API for simple external access, with an authentication scheme built on top of it. These heavily abstracted interfaces can allow several entirely tangent applications to consume and alter data in a manner consistent with their implementation, while maintaining a single distinct data source.
It also can allow for the distribution of the total request load across several servers, which simply have to be aware of the API implementation and a data source.

Conclusion

REST is an important architectural model that defines itself as a set of five or six restrictions on top of an “unbounded” architecture. REST is not any one implementation, or any one concept or use case. It’s a highly extensible architecture that drives the web as we know it today, but is independent of its implementation in HTTP.

Many descriptions of REST are overly academic or too specific to a single implementation. I hope that I’ve provided a good resource on the fundamental meaning and purpose of REST, independent of HTTP, as well as its use in HTTP and web services today.

Given that this is a complex topic, on which all information essentially traces itself back to that single dissertation, it’s possible that some information may be inaccurate, so please let me know if you find these types of mistakes and they will be corrected immediately.