Enabling truly easy testability with ASP.NET MVC Controllers requires a Dependency Injection container. This post is about showing a more real life example of combining Unity with ASP.NET MVC Controllers than the standard demo. Unity is the IoC Container from Microsoft Patterns & Practices and ships as a part of Enterprise Library as well as a stand alone on Codeplex.

We start with the unit test which drives the design of the controller.

Even though the test becomes a bit verbose it clearly states the intent of the controller’s Post method; given the right slug, a result is returned with a blog post as model.

Dependency Injection in ASP.NET MVC

In ASP.NET MVC every request starts in a controller which uses a set of dependencies to do its job. When we have applied Inversion of Control we can inject all of the controller’s dependencies either as a constructor argument or as a property setter. To fully take advantage of this technique we want the dependencies to be automatically injected without creating any of them in overloaded constructors or something like that. This is where the Controller Factory in ASP.NET MVC comes into play.

Controller Factory

In the System.Web.Mvc namespace lives the IControllerFactory which implementation is used by the framework to create controller instances. We can use this to swap out the default one and replace it with our own controller factory which uses Unity to create controllers instead.

Instead of implementing all of the IControllerFactory functionallity we can extend the System.Web.Mvc.DefaultControllerFactory and only override what we need.

In the Global.asax, called MvcApplication in ASP.NET MVC by default, we create our Unity container and tells the MVC framework to use our UnityControllerFactory instead of the default one.

PostController

To illustrate why this is so powerful we will implement a controller that handles requests for blog posts, such as the one you’re reading right now. For this example the controller only handles requests for single blog posts using the post’s slug as identity. A slug is the post’s title made url friendly (e.g. "Dependency-injection-in-ASPNET-MVC-with-Unity-IoC-Container” for this post). The PostController has a dependency on an IBlogPostService which it uses to lookup the blog post to send to the view.

Strongly-typed views

The Post action returns an ActionResult with the post a the model. We can use strongly-typed views to get intellisense in the aspx-file.

<%@ Page /*...*/ Inherits="ViewPage" %>

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

    <%=Model.Title %>

asp:Content>

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">

    <h2><%=Model.Title %>h2>

    <p>

        <%=Model.Body %>

    p>

    <p><%=Html.PostActionLink("Permalink", Model) %>p>

    <p><a href="/2009/10/10/fail">fail linka>p>

asp:Content>

Notice how the view inherits from ViewPage which is key to make the model type of IBlogPost. Then we can use the Post properties to render the blog post in the view.

Conclusion

Combining ASP.NET MVC with Unity or your IoC Container of choice makes your controllers much more easily testable. You only state your classes dependencies as constructor parameters or properties and the container will do the wiring for you. The gives you time to focus on the program logic and the interaction between its components.