Injecting dependencies into ASP.NET MVC 3 action filters. What's wrong with this approach?

Question

Here's the setup. Say I have some action filter that needs an instance of a service:

public interface IMyService
{
   void DoSomething();
}

public class MyService : IMyService
{
   public void DoSomething(){}
}

I then have an ActionFilter that needs an instance of that service:

public class MyActionFilter : ActionFilterAttribute
{
   private IMyService _myService; // <--- How do we get this injected

   public override void OnActionExecuting(ActionExecutingContext filterContext)
   {
       _myService.DoSomething();
       base.OnActionExecuting(filterContext);
   }
}

In MVC 1/2 injecting dependencies into action filters was a bit of a pain in the ass. The most common approach was to use a custom action invoker as can be seen here: http://www.jeremyskinner.co.uk/2008/11/08/dependency-injection-with-aspnet-mvc-action-filters/ The main motivation behind this workaround was because this following approach was considered sloppy and tight coupling with the container:

public class MyActionFilter : ActionFilterAttribute
{
   private IMyService _myService;

   public MyActionFilter()
      :this(MyStaticKernel.Get<IMyService>()) //using Ninject, but would apply to any container
   {

   }

   public MyActionFilter(IMyService myService)
   {
      _myService = myService;
   }

   public override void OnActionExecuting(ActionExecutingContext filterContext)
   {
       _myService.DoSomething();
       base.OnActionExecuting(filterContext);
   }
}

Here we're using constructor injection and overloading the constructor to use the container and inject the service. I do agree that does tightly couple the container with the ActionFilter.

My question though is this: Now in ASP.NET MVC 3, where we have an abstraction of the container being used (through the DependencyResolver) are all these hoops still necessary? Allow me to demonstrate:

public class MyActionFilter : ActionFilterAttribute
{
   private IMyService _myService;

   public MyActionFilter()
      :this(DependencyResolver.Current.GetService(typeof(IMyService)) as IMyService)
   {

   }

   public MyActionFilter(IMyService myService)
   {
      _myService = myService;
   }

   public override void OnActionExecuting(ActionExecutingContext filterContext)
   {
       _myService.DoSomething();
       base.OnActionExecuting(filterContext);
   }
}

Now I know that some purists might scoff at this, but seriously, what would be the downside? It's still testable as you can use the constructor that takes an IMyService at test time and inject a mock service that way. You're not tied down to any implementation of DI container since you're using the DependencyResolver, so are there any downsides to this approach?

Incidentally, here's another nice approach for doing this in MVC3 using the new IFilterProvider interface: http://www.thecodinghumanist.com/blog/archives/2011/1/27/structuremap-action-filters-and-dependency-injection-in-asp-net-mvc-3

Answer 1

Yes, there are downsides, as there are lots of issues with IDependencyResolver itself, and to those you can add the use of a Singleton Service Locator, as well as Bastard Injection.

A better option is to implement the filter as a normal class into which you can inject whichever services you'd like:

public class MyActionFilter : IActionFilter
{
    private readonly IMyService myService;

    public MyActionFilter(IMyService myService)
    {
        this.myService = myService;
    }

    public void OnActionExecuting(ActionExecutingContext filterContext)
    {
        if(this.ApplyBehavior(filterContext))
            this.myService.DoSomething();
    }

    public void OnActionExecuted(ActionExecutedContext filterContext)
    {
        if(this.ApplyBehavior(filterContext))
            this.myService.DoSomething();
    }

    private bool ApplyBehavior(ActionExecutingContext filterContext)
    {
        // Look for a marker attribute in the filterContext or use some other rule
        // to determine whether or not to apply the behavior.
    }

    private bool ApplyBehavior(ActionExecutedContext filterContext)
    {
        // Same as above
    }
}

Notice how the filter examines the filterContext to determine whether or not the behavior should be applied.

This means that you can still use attributes to control whether or not the filter should be applied:

public class MyActionFilterAttribute : Attribute { }

However, now that attribute is completely inert.

The filter can be composed with the required dependency and added to the global filters in global.asax:

GlobalFilters.Filters.Add(new MyActionFilter(new MyService()));

---

For a more detailed example of this technique, although applied to ASP.NET Web API instead of MVC, see this article: http://blog.ploeh.dk/2014/06/13/passive-attributes