Introducing ASP.NET MVC (Model View Controller)
by Michael Kennedy

There's big news on the horizon for ASP.NET! Scott Guthrie and team have just released the first preview of a much anticipated and entirely new application framework for ASP.NET: a true model-view-controller framework for ASP.NET called ASP.NET MVC.

This is the most significant change to the programming model for ASP.NET since its introduction in 2002 with .NET 1.0. The ASP.NET team, learning from the tough lessons of the past, made the decision that this new MVC programming model will not replace the current models, but will instead be an additional application model for ASP.NET available in Visual Studio 2008 and beyond. This will increase your options for building websites in ASP.NET, providing now three different application models to choose from:

What is MVC?
The model-view-controller design, for those not familiar with it, is an architectural methodology that was introduced with Smalltalk (way back in 1979!) and splits an application into three separate layers:

(M)odel - the data being manipulated. This is typically wrapped with domain-specific logic and includes some form of data access layer.

(V)iew - the presentation of the data (UI).

(C)ontroller - the connection between the model and view which responds to events and triggers actions in the model.

The MVC pattern has been used successfully in building highly scalable websites for years: the HTML is the view, the controller is the code generating the HTML and the data is typically XML or some persistent storage holding the raw information being rendered.

Why formally add it to ASP.NET?
Up to now, programmers have been forced to build a significant amount of code around the raw ASP.NET engine to utilize the MVC design pattern. With the introduction of this new programming model, you can now skip all that work and build directly on an extensible framework provided by Microsoft!

Learning a new framework is a challenge, so it’s useful to look at what this MVC model will provide over the existing models you already know and love in ASP.NET:

In addition to these fundamental design principles, MVC has deep support for existing ASP.NET concepts such as master pages, themes, membership providers, etc.

Architecture
The architecture of ASP.NET MVC is fairly straightforward however, the details of how the pieces are wired together takes some exploring. When implemented correctly, the ASP.NET MVC architecture should strongly resemble Martin Fowler's Passive View design pattern.

First, let's look at the major components of the framework as we trace a request from start to finish.

To stay true to the Passive View design pattern, the view should not interact with the model or other business layer components. It must only use the DTO data to generate HTML. This can take some diligence when first programming with MVC.

Understanding the Project Structure
Now it's time to see this architecture reflected in the Visual Studio project structure and discuss how the controllers and views are tied together.



There are three top level folders.

In the controllers folder, we see classes that are the controllers, e.g. PlaceOrderController. In the views folder, we have a little more structure. Each controller has a subfolder, e.g. Views/PlaceOrder. Within that folder, we have the views used by the PlaceOrder controller. There is also a Views/Shared folder for things like master pages.

A Little Less Talk and A Lot More Action - ControllerAction That Is
Let's put it all together by defining a controller, controller action, and render the result in a view.

First we define a controller action to map to /PlaceOrder/ViewCart.



Notice the ControllerAction attribute, reminiscent of WCF and ASP.NET Web Services. The action computes the information needed by the view to display the current cart contents, then creates a DTO (ViewCartData) to pass to the view which is done by calling RenderView.

The naming conventions tie this together in the framework: when the PlaceOrderController class calls RenderView("ViewCart"), MVC routes the call to the Views/PlaceOrder/ViewCart.aspx template.

On the view side we have two parts, the template and the code-behind. Both should only interact with the DTO to generate the final HTML for the UI. Notice in the code behind we inherit from ViewPage<ViewCartData>. This maps the DTO to this.ViewData on the view.

View Template:
 

View CodeBehind: 



Conclusion
ASP.NET MVC is an interesting new application architecture for ASP.NET. It enables a new class of web applications - well factored and test driven. The current release is just a CTP (Community Technology Preview) and will likely change as it evolves, but keep your eye on MVC -- it's expected to be released before the next version of Visual Studio.

One final note: If you want to try ASP.NET MVC for yourself, you need to have VS 2008 installed and additionally install the ASP.NET 3.5 Extensions Preview. Then just choose File->New Project->Web and you're on your way!

To contact Michael Kennedy, please send email to: michaelk@develop.com

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