The current mechanism for performing property injection on FilterAttribute instances via the ExtensibleActionInvoker had to be removed recently due to a rather nasty bug. These are the notes that Nick provided outlining the problem he discovered (possibly with the help of the exciting new Whitebox profiler).

Because the filters passed from the base action invoker also include the controller, property injection happens on the controller itself several times as the filters are processed.

The filter attributes also included in the collection may also be singletons cached by MVC, and so it is quite likely that dependencies may be overwritten with those from a concurrently executing request.

In all this behaviour is probably too risky to reliably support.

Removed property injection routine. (Breaking change.)

I have replaced the old mechanism using an approach that leverages the improved dependency injection support added to MVC 3 (this will be in the next release). To make use of property injection for your filter attributes all you will need to do is call the RegisterFilterProvider method on the ContainerBuilder before building your container and providing it to the AutofacDependencyResolver.

ContainerBuilder builder = new ContainerBuilder();

builder.RegisterControllers(Assembly.GetExecutingAssembly());
builder.Register(c => new Logger()).As<ILogger>().InstancePerHttpRequest();
builder.RegisterFilterProvider();

IContainer container = builder.Build();
DependencyResolver.SetResolver(new AutofacDependencyResolver(container));

Then you can add properties to your filter attributes and any matching dependencies that are registered in the container will be injected into the properties. For example, the action filter below will have the ILogger instance that was registered above injected. Note that the attribute itself does not need to be registered in the container.

public class CustomActionFilter : ActionFilterAttribute
{
    public ILogger Logger { get; set; }

    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        Logger.Log("OnActionExecuting");
    }
}

The same simple approach applies to the other filter attribute types such as authorization attributes.

public class CustomAuthorizeAttribute : AuthorizeAttribute
{
    public ILogger Logger { get; set; }

    protected override bool AuthorizeCore(HttpContextBase httpContext)
    {
        Logger.Log("AuthorizeCore");
        return true;
    }
}

After applying the attributes to your actions as required your work is done.

[CustomActionFilter]
[CustomAuthorizeAttribute]
public ActionResult Index()
{
    // ...
}

To make this work I added a custom FilterAttributeFilterProvider implementation. The custom filter provider delegates the job of collecting the filters to the base class. Once the filters have been retrieved by the base class, the ILifetimeScope for the current HTTP request is retrieved and used to perform property injection on the filters. The false passed to the base FilterAttributeProvider constructor sets the cacheAttributeInstances parameter to ensure that attribute instances are not cached. Allowing the attribute instances to be cached would result in race conditions and other unexpected behaviour.

/// <summary>
/// Defines a filter provider for filter attributes that performs property injection.
/// </summary>
public class AutofacFilterAttributeFilterProvider : FilterAttributeFilterProvider
{
    /// <summary>
    /// Initializes a new instance of the <see cref="AutofacFilterAttributeFilterProvider"/> class.
    /// </summary>
    /// <remarks>
    /// The <c>false</c> constructor parameter passed to base here ensures that attribute instances are not cached.
    /// </remarks>
    public AutofacFilterAttributeFilterProvider() : base(false)
    {
    }

    /// <summary>
    /// Aggregates the filters from all of the filter providers into one collection.
    /// </summary>
    /// <param name="controllerContext">The controller context.</param>
    /// <param name="actionDescriptor">The action descriptor.</param>
    /// <returns>
    /// The collection filters from all of the filter providers with properties injected.
    /// </returns>
    public override IEnumerable<Filter> GetFilters(ControllerContext controllerContext, ActionDescriptor actionDescriptor)
    {
        var filters = base.GetFilters(controllerContext, actionDescriptor).ToArray();
        var lifetimeScope = AutofacDependencyResolver.Current.RequestLifetimeScope;

        if (lifetimeScope != null)
            foreach (var filter in filters)
                lifetimeScope.InjectProperties(filter.Instance);

        return filters;
    }
}

The RegisterFilterProvider method has been added to the ContainerBuilder using an extension method. This method will register the AutofacFilterAttributeFilterProvider using the IFilterProvider interface that MVC uses when asking the dependency resolver for filter providers. Following the instructions outlined in Brad Wilson’s post on the subject of dependency injection and filters, I made sure that the default FilterAttributeFilterProvider instance is removed from the static collection of providers.

/// <summary>
/// Registers the <see cref="AutofacFilterAttributeFilterProvider"/>.
/// </summary>
/// <param name="builder">The container builder.</param>
public static void RegisterFilterProvider(this ContainerBuilder builder)
{
    if (builder == null) throw new ArgumentNullException("builder");

    foreach (var provider in FilterProviders.Providers.OfType<FilterAttributeFilterProvider>().ToArray())
        FilterProviders.Providers.Remove(provider);

    builder.RegisterType<AutofacFilterAttributeFilterProvider>()
        .As<IFilterProvider>()
        .SingleInstance();
}

If you were using the old mechanism you will have breaking changes to contend with, but as you can see it should be easy to get back on track again.

The increased support for dependency injection in ASP.NET MVC 3 includes the ability to have your view pages created by your favourite container.

Historically, these classes have not had access to dependency injection/service location functionality, because their creation was buried deep inside the implementation of the view engine. In MVC 3, we have updated the built-in view engines to attempt to create the view page classes via the service locator; if that fails, it will fall back to using Activator.CreateInstance, just like in previous versions of MVC.

Because the view pages are dynamically compiled at runtime a few restrictions have been imposed; you cannot use constructor injection and your view pages must inherit from a custom base class.

The problem is that your .aspx/.ascx/.cshtml/.vbhtml files are converted into classes at runtime by the ASP.NET Build Manager (in collaboration with build providers). When those classes are auto generated, they are auto generated only with a single parameterless constructor.

We looked at auto-generating constructors, but it turns out that we don't actually know enough about the base class when we're generating the code to actually do any reflection on it, so it's not really possible for us to look at the base class and determine which constructors it may or may not have.

Happy that these limitations are not going to pose any serious problems let’s move onto the Autofac integration. Time for yet another uninspiring example, but one that should be easy to follow and doesn’t require too much typing on my part. Imagine that we have a service that provides common company information such as a copyright that we need to display on all our view pages.

public interface ICompanyInformation
{
    string Copyright { get; }
}

There is of course an implementation of the service that returns the dynamic copyright information (you were warned about the example).

public class CompanyInformation : ICompanyInformation
{
    public string Copyright
    {
        get { return string.Format("Copywrong © {0} ACME Corporation", DateTime.Now.Year); }
    }
}

In the application start event we build our container and register the service along with our controllers. We also add a registration source called ViewRegistrationSource.

ContainerBuilder builder = new ContainerBuilder();
builder.Register(c => new CompanyInformation()).As<ICompanyInformation>().InstancePerHttpRequest();
builder.RegisterControllers(Assembly.GetExecutingAssembly());
builder.RegisterSource(new ViewRegistrationSource());

IContainer container = builder.Build();
DependencyResolver.SetResolver(new AutofacDependencyResolver(container));

The registration source is where all the magic happens. A registration source allows you to create an adapter that will dynamically provide a registration for a service. We know that MVC will ask the container for an instance of the view page before it attempts to create it itself, so we can use the registration source to make sure that the container always knows how to provide such an instance. Below is the implementation of the registration source for those that are interested in the details.

public class ViewRegistrationSource : IRegistrationSource
{
    public IEnumerable<IComponentRegistration> RegistrationsFor(Service service, Func<Service, IEnumerable<IComponentRegistration>> registrationAccessor)
    {
        var typedService = service as IServiceWithType;

        if (typedService != null && IsSupportedView(typedService.ServiceType))
            yield return RegistrationBuilder.ForType(typedService.ServiceType)
                .PropertiesAutowired()
                .InstancePerHttpRequest()
                .CreateRegistration();
    }

    public bool IsAdapterForIndividualComponents
    {
        get { return false; }
    }

    static bool IsSupportedView(Type serviceType)
    {
        return serviceType.IsAssignableTo<WebViewPage>() 
            || serviceType.IsAssignableTo<ViewPage>()
            || serviceType.IsAssignableTo<ViewMasterPage>()
            || serviceType.IsAssignableTo<ViewUserControl>();
    }
}

If the requested service inherits from one of the supported view base classes, the RegistrationBuilder.ForType helper is used to build the registration. The registration also makes sure that property injection is performed and that the lifetime is scoped to the HTTP request. The Razor view base class WebViewPage is supported, along with the WebForms base classes ViewPage, ViewMasterPage and ViewUserControl.

To get properties on the view page that can be injected by the container, you need to slot your own base class into the inheritance hierarchy. This is as simple as creating an abstract class that derives from WebViewPage or WebViewPage<T> when using the Razor view engine.

public abstract class CustomViewPage : WebViewPage
{
    public ICompanyInformation CompanyInformation { get; set; }
}

If you are using the WebForms view engine in your MVC project you would derive from the ViewPage or ViewPage<T> class instead.

public abstract class CustomViewPage : ViewPage
{
    public ICompanyInformation CompanyInformation { get; set; }
}

The last thing you need to do is ensure that your actual view page inherits from your custom base class. This can be achieved using the @inherits directive inside your .cshtml file for the Razor view engine.

@inherits Example.Views.Shared.CustomViewPage

@{
    ViewBag.Title = "Home Page";
}

<h2>@ViewBag.Message</h2>
<p>
    This is the page content.
</p>
<p>
    @CompanyInformation.Copyright
</p>

When using the WebForms view engine you set the Inherits attribute on the @ Page directive inside you .aspx file instead.

<%@ Page Language="C#" MasterPageFile="~/Views/Shared/Site.Master" Inherits="Example.Views.Shared.CustomViewPage"%>

<asp:Content ID="Content1" ContentPlaceHolderID="TitleContent" runat="server">
    Home Page
</asp:Content>

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
    <h2><%: ViewBag.Message %></h2>
    <p>
        This is the page content.
    </p>
    <p>
        <%= CompanyInformation.Copyright %>
    </p>
</asp:Content>

Making your custom base class inherit from the generic WebViewPage<T> or ViewPage<T> class allows you to provide your strongly typed model as the generic type parameter. You can of course choose to leave the generic type parameter in your base class open making it more reusable.

public abstract class CustomViewPage<T> : WebViewPage<T>
{
    public ICompanyInformation CompanyInformation { get; set; }
}

You simply provide the model type as the closing generic parameter in the type declared in the @inherits or Inherits attribute of the page.

@inherits Example.Views.Shared.CustomViewPage<Example.Models.CustomModel> 

Taking advantage of view page injection is a very simple matter. No doubt you will have much more creative uses for this than the simplified example shown here.

I have just checked into trunk a first pass at the ASP.NET MVC 3 Beta integration for Autofac. In hope of simplifying the requirements for those getting started with the integration I wanted to prevent the need to:

• Add a reference the Autofac.Integration.Web.dll assembly
• Implement the IContainerProviderAccessor interface on the HttpApplication
• Register the ContainerDisposalModule in the web.config file

The code below is an example of all you would need to put into the application start event to get up and running.

ContainerBuilder builder = new ContainerBuilder();
builder.RegisterControllers(Assembly.GetExecutingAssembly());
builder.Register(c => new Logger()).As<ILogger>().InstancePerHttpRequest();

IContainer container = builder.Build();
DependencyResolver.SetResolver(new AutofacDependencyResolver(container));

The core piece of the integration is the AutofacDependencyResolver. This is an implementation of the IDependencyResolver interface that Brad Wilson outlines in his blog post series on ASP.NET MVC 3 Service Location. The interface requires you to implement two simple methods: GetService and GetServices. When no service is found GetService should return null, and GetServices should return an empty IEnumerable<object>. The implementation of these methods ends up being straight forward (ignoring for now the code related to managing the CurrentLifetimeScope).

public object GetService(Type serviceType)
{
    return CurrentLifetimeScope.IsRegistered(serviceType) ? CurrentLifetimeScope.Resolve(serviceType) : null;
}

public IEnumerable<object> GetServices(Type serviceType)
{
    Type enumerableServiceType = typeof(IEnumerable<>).MakeGenericType(serviceType);
    object instance = CurrentLifetimeScope.Resolve(enumerableServiceType);
    return ((IEnumerable)instance).Cast<object>();
}

When MVC needs to create a controller it will ask the DependencyResolver for an instance. The AutofacDependencyResolver returns the controllers that are registered in the container it was provided. These are usually registered using the RegisterControllers method on the ContainerBuilder as shown in the first code sample. There is no longer a need to create a class that derives from the DefaultControllerFactory for the sole purpose of returning controller instances. This means the AutofacControllerFactory is no longer required and has been removed.

The Autofac MVC integration has always supported the concept of a HTTP request lifetime scope. This means that the lifetime of a service can be scoped to the current HTTP request. The ILogger service registration in the sample code above uses the InstancePerHttpRequest method to indicate that the same instance of the logger service should be used for all dependency resolutions that occur during the current HTTP request. To make sure that the nested lifetime scope that Autofac creates for each request is disposed, it needs to be notified when the request has ended.

The only reliable way to do this is to create a HTTP module that subscribes to the EndRequest event of the HttpApplication. To register a HTTP module you need to add an entry to the web configuration file, which is something that I was hoping to avoid. Rick Strahl outlines one way of achieving programmatic registration of a module in his Dynamically hooking up HttpModules post, but for the integration this would require the user to manually add the code to their HttpApplication instance (by default called MvcApplication).

It turns out that there is in fact another way to programmatically register a module. The Microsoft.Web.Infrastructure.dll assembly that ships with the ASP.NET Web Pages installer (AspNetWebPages.msi) contains a rather helpful class called DynamicModuleUtility. It has a single method called RegisterModule that accepts a Type for the module to register. You can only call this helper from a method that is marked as pre application start code as defined by the PreApplicationStartMethodAttribute applied to an assembly. The same trick is used in System.Web.Pages.dll to register the new WebPageHttpModule. Phil Haack has a blog post that talks about the PreApplicationStartMethodAttribute and some other interesting new ASP.NET 4 features in greater detail if you are keen to know more. You need to install ASP.NET Web Pages before installing ASP.NET MVC 3 so we know the assembly with this helpful little gem will be available.

In the Autofac integration we first needed to add the assembly attribute.

[assembly: PreApplicationStartMethod(typeof(PreApplicationStartCode), "Start")]

This points to a static class that contains a single static method called Start. Inside this method we call the DynamicModuleUtility to register the RequestLifetimeModule that will informs us when the HTTP request has ended. There is no need to ever call this class directly but unfortunately, it and the method must be public. That is why we have the EditorBrowsable attribute being applied in order to hide the class from the editor. Not really that much work to save a user from having to dive into the web configuration file.

[EditorBrowsable(EditorBrowsableState.Never)]
public static class PreApplicationStartCode
{
    private static bool _startWasCalled;

    public static void Start()
    {
        if (_startWasCalled) return;

        _startWasCalled = true;
        DynamicModuleUtility.RegisterModule(typeof(RequestLifetimeModule));
    }
}

There is a new interface in the MVC 3 integration called ILifetimeScopeProvider. The HTTP module RequestLifetimeModule shown above actually implements this interface and is the default implementation used by the AutofacDependencyResolver. You can see from the AutofacDependencyResolver code shown at the start of the post that the resolutions are happening from the CurrentLifetimeScope property.

internal ILifetimeScope CurrentLifetimeScope
{
    get
    {
        if (_lifetimeScopeProvider == null)
            _lifetimeScopeProvider = GetRequestLifetimeModule();
        return _lifetimeScopeProvider.GetLifetimeScope(_container, _configurationAction);
    }
}

You can add your own ILifetimeScopeProvider implementation to the container that is passed to the AutofacDependencyResolver if you want to replace the HTTP request based lifetime behaviour. The AutofacDependencyResolver will attempt to retrieve it from the container during its constructor. Because the RequestLifetimeModule is the default ILifetimeScopeProvider and an instance was already created by ASP.NET when the module was initialised, we can go and grab that from the HttpModuleCollection of the current HttpApplication.

static ILifetimeScopeProvider GetRequestLifetimeModule()
{
    HttpModuleCollection httpModules = HttpContext.Current.ApplicationInstance.Modules;
    for (int index = 0; index < httpModules.Count; index++)
    {
        if (httpModules[index] is RequestLifetimeModule)
            return (RequestLifetimeModule)httpModules[index];
    }
    throw new InvalidOperationException(string.Format(
        AutofacDependencyResolverResources.HttpModuleNotLoaded, typeof(RequestLifetimeModule)));
}

None of the model binding code has been moved into the new integration yet. I am hoping that this can be refactored to use the new IModelBinderProvider interface. This is only a first pass based on a new approach so it is likely that some of this will change. I have certainly found the exercise interesting enough that I thought it was worth sharing the start of the journey.

There are currently two main approaches to performing dependency injection, Constructor Injection and Setter Injection. The more popular of the two approaches is Constructor Injection. The dependencies that a type has are made obvious because they must be supplied in order to construct an instance. This also makes it easier for you to ensure that a newly instantiated object is in a valid state. When working with a type the constructor is usually the first thing that you come into contact with.

With Setter Injection, also known as Property Injection, it is much more difficult to tell what the dependencies are when looking at the type from the outside. Setter Injection is most useful when you have no control over the instantiation of the type that requires the dependencies to be injected. This is a common scenario for ASP.NET WebForms where the activation of a Page instance is performed by the runtime. You do not have an opportunity to take over the activation process, and the first chance you have to perform dependency injection is when you are provided with an existing instance of Page. In this case you have no choice but to inject the dependencies into the type via its properties.

ASP.NET MVC has many extensibility points and is very flexible. It provides you with the opportunity to take over the creation of your Controller instances by creating your own factory that implements IControllerFactory, or more commonly by deriving from the DefaultControllerFactory and overriding the GetControllerInstance method. This makes it possible for your controllers to take advantage of Constructor Injection, and is exactly what the Autofac ASP.NET MVC Integration does. When it comes to unit testing your controller classes, it becomes very easy to see what dependencies it has, and to provide mock implementations for those dependencies.

An issue that is often raised in regards to Constructor Injection is what some people like to call Constructor Bloat. This may indicate that you are not following the Single Responsibility Principle and that some refactoring may be in order. The number of constructor parameters that would be considered too many would no doubt vary depending on who you ask. In the case of ASP.NET MVC controllers the number of constructor dependencies is more likely to be higher than for other classes. The level of responsibility for a controller is usual greater than what you would expect for an ordinary internal component. This is the result of mapping an external view of the application (URL based) onto an internal representation (controller based).

It turns out that both Nicholas Blumhardt and I found ourselves shifting some of these dependencies out of the controller’s constructor and into the action methods that actually require them. We were both fairly surprised to find out that the other had independently been doing exactly the same thing, and at this point discussed if there was something wrong with the approach because it seemed that no one else was doing it. Surely all good ideas have already been done so this one must be bad. I personally feel that having dependencies injected into your action method should not feel like a foreign concept because that is exactly what MVC is already doing for you with your existing parameters.

For lack of any official term that I am aware of, Action Injection is what I am calling this particular approach to dependency injection in ASP.NET MVC. The more I play around with this approach the more I like it. Your constructor is provided the dependencies that are shared by all actions in your controller, and each individual action can request any additional dependencies that it needs. Now when writing unit tests for your actions there is no need to provide mock implementations for dependencies that your action will not be interacting with. The end result is less mocks in your unit tests and a clear indication of the action’s actual dependencies.

Nick and I have decided to test out the idea of Action Injection in the Autofac ASP.NET MVC Integration. The changes are only in the source code at the moment and have not yet been included in a release. I mentioned earlier that MVC is very extensible and the process for invoking your action methods is no different. It is possible to replace the default behaviour by creating your own IActionInvoker. The easiest way to do this is by deriving from the AsyncControllerActionInvoker class and overriding the appropriate methods. A controller can be requested to use your custom action invoker by assigning an instance to the controller's ActionInvoker property. The current source includes a registration extension that allows you to register an IActionInvoker instance that will be assigned to a controller as it is activated. There is a default IActionInvoker implementation called ExtensibleActionInvoker that allows dependencies to be injected into your action methods. It can also do Setter Injection on your filters but that is a topic for another post. As the name suggests, you can extend this class and add any additional behaviour that you require. Registering controllers in the HttpApplication start would look something like this.

ContainerBuilder builder = new ContainerBuilder();

builder.RegisterType<ExtensibleActionInvoker>()
    .As<IActionInvoker>()
    .WithParameter("injectActionMethodParameters", true);
builder.RegisterControllers(Assembly.GetExecutingAssembly())
    .InjectActionInvoker();

// Register other services.

IContainer container = builder.Build();
_containerProvider = new ContainerProvider(container);

ControllerBuilder.Current.SetControllerFactory(new AutofacControllerFactory(_containerProvider));

I will not go into further detail on the implementation at this point because it may be tweaked a little before being released. Instead, let us look at an example of how we could make our action dependencies clearer using Action Injection. The NotifyController class below has action methods that send the current user a message using different delivery methods.

public class NotifyController
{
    public NotifyController(ILogger logger, 
        IEmailNotifier emailNotifier, 
        ISmsNotifier smsNotifier, 
        IMessengerNotifier messengerNotifier)
    {
        // Implementation.
    }
    
    public ActionResult Email(string message)
    {
        // Implementation.
    }
    
    public ActionResult Sms(string message)
    {
        // Implementation.
    }

    public ActionResult Messenger(string message)
    {
        // Implementation.
    }
}

There are three action methods on this controller and four dependencies that must be provided through the constructor. To unit test any of the action methods all four of the dependencies will need to be mocked. In this controller the ILogger instance is required by all action methods, but the remaining notifier dependencies are each required only by one action method. The controller could be refactored so that it takes the one ILogger dependency through its constructor, and each action could take its particular notifier dependency through a method parameter. Here is an example of how the refactored code would look.

public class NotifyController
{
    public NotifyController(ILogger logger)
    {
        // Implementation.
    }
    
    public ActionResult Email(string message, IEmailNotifier emailNotifier)
    {
        // Implementation.
    }
    
    public ActionResult Sms(string message, ISmsNotifier smsNotifier)
    {
        // Implementation.
    }

    public ActionResult Messenger(string message, IMessengerNotifier messengerNotifier)
    {
        // Implementation.
    }
}

Now when testing the action methods we only ever need to provide two mock services. There is no need to provide additional mock services that will never be used. Assuming we only had one unit test per action and setup our mocks inside each unit test, we would have halved the number of mocks required, taking the total from twelve down to six. That certainly seems like an improvement to me.

I would be interested to know what you think about this idea. Is it totally crazy or could there be something to it? Maybe you too have already been doing this and could share how it has been working out for you.

It looks like ASP.NET MVC 1.0 has been released and is available for download. Microsoft have been very open with the development of MVC and have really taken community feedback on board. The end result of taking this transparent approach speaks for itself.

Phil Haack from Microsoft has written about the release of MVC and I’m sure ScottGu will soon roll out another one of his monster blog posts. It will be interesting to see what Microsoft has in store for the next version of MVC.