Tag Archives: MVC blog

Data Annotations

One of the many benefits of separating the concerns of the models, views, and controllers in an MVC application is that each of the three components is able to contain all of the code with which it is concerned. This means, for example, that the view is only concerned with the user interface, and contains only the code that displays a page and data to the user. It doesn’t have any business logic, data access, or any other code unrelated to the user interface.

In the same way, an MVC model is concerned only with data and business logic, nothing else. It interacts with a data store, such as SQL Server, and takes care of all the CRUD operations for maintaining data. In theory, this helps maintain a clean separation of concerns with the other MVC components.

But not everything is always so black and white or neatly partitioned. One issue that would seem like it crosses the component boundary is data validation. On one side, a relational database has certain requirements for data, such as a ProductID that is an integer, and that a company name is a string of one to fifty characters, no more and no less. It may also mandate that every product be assigned to a non-null product category, and that a U.S. Zip code be either five or nine characters in length.

Beyond those data type sort of requirements, you may also have business logic that requires that an email address be of a format that meets the requirements of the Internet, or that no customer can receive more than five orders a year with greater than a 20% discount. All of these data and business logic rules are in place to ensure that stored data is of the highest quality and that the application doesn’t allow any violations of the rules by which the business operates.

But where and how should you define these rules? On one hand, they are the domain of the model because most underlying data stores have requirements that must be met to store data. And because an MVC model is the proper location for business rules, it seems clear that they go in the model as well.

On the other hand, however, it is usually the user interface that needs to take a hand in enforcing the rules and helping the user provide the correct data by displaying error messages when she tries to save data that is missing a field. An input field of type text for the company name might have the max length attribute set to 50, to prevent the user from even entering too many characters in the first place. The user interface clearly has a role to play in data validation.

So where should you implement data validation, in the view or the model? The good news is that with an MVC model there is a clear answer: the model. You add attributes to the properties and classes that implement the model that define the data validation rules, and you can add other data annotations that guide how the view should use the properties of the model. The view can then look at those rules and annotations, and then enforce the rules and appropriately modify how it displays the data to the user. The view’s role in data validation is to take the rules defined in the model—without defining any of its own—and provide a user interface that enforces those rules.

Best of all, since the rules are part of the model, they’ll be in effect no matter where you use the model, across all views and controllers. There is no need to duplicate the rules anywhere in the application. This is an application of the DRY principle of an MVC application: Don’t Repeat Yourself.

James Curtis

This post is an excerpt from the online courseware for our MVC 4.0: Working with Data course written by expert James Curtis.

James Curtis  is a .NET Developer that primarily works in the UX space. He has worked on and for several large projects alongside Microsoft Consulting. James has spoken at several code camps about UX development for ASP.NET and SharePoint. He is an active participant in the development community tweeting and blogging about several topics in the UX area. James is an active consultant and is also assisting in several Start-ups contributing his UX experience.

MVC 4.0: The Role of the View

The views in your ASP.NET MVC application are what a user sees. Views are the window into your application, and so is one of the most important parts of the application…along with all the rest of it!

Views provide one of the biggest reasons to use MVC instead of Web Forms. Unlike Web Forms, you have complete control over the HTML that the application generates. There are no server controls as there are in Web Forms that spew HTML, which you have little control over or that are hard to use with CSS and client-side JavaScript.

Having complete control over HTML sounds great in theory, but it could also mean that you’d have to write a lot more HTML by hand. One of the big selling points of Web Forms is that you can do drag and drop development, using server controls that generate massive amounts of HTML based on the properties you set and the data you bind to the controls.

But MVC has a lot of features built in that make the job of writing HTML and other code simpler and more manageable, while letting you retain control over most of the generated HTML. And when you need complete control, you can bypass the MVC features and write all the beautiful HTML you care to write.

The trick to writing good views in MVC is to keep the view focused on the user interface only. This means that there should be no code that directly reads or writes to a data store, no business rule implementations, nothing that isn’t directly related to interacting with the user. The ideal view should consist of a lot of HTML, maybe some client-side JavaScript (perhaps including Ajax), and a little code that reads data from the model or ViewData for the purpose of displaying data to the user. It might be appropriate to put some logic in a view, but it should only be presentation logic needed to dynamically interact with the user. Anything else you should rip out and put in the controller or model. Make that your solemn goal.

The purest way to think about views is that the controller hands a model to the view, and the view converts it into something that is presented to the user. We’ve found this way of thinking to be a helpful context for understanding what views do and how to create them, as well as where to implement different kinds of logic.

In this chapter, you’ll learn about how views work, how you can add dynamic data to the view, how you can bundle and minify files, a little bit about the view engine that makes it all work, and ways to adhere to MVC’s DRY—Don’t Repeat Yourself—principle by creating reusable controls that you can use in multiple views.

The Figure below shows a graphic view of the interactions of the components of an MVC application. In order to handle a user request, the controller interacts with the model in whatever way is necessary to respond to the request. The controller then selects a view and passes it whatever data the view requires to generate a page. The view engine then reads the view template and dynamically generates the HTML that is sent to the user.

MVC4 Views
The Figure above. Anatomy of an MVC request and response.

In this chapter, you’ll learn some of the many features you can use to build views that provide a robust user interface for your applications. The MVC framework has a rich infrastructure for creating responsive and attractive web sites for your users.


ldn-pledgerwoodThis post is an excerpt from the online courseware for our MVC 4.0: Views and Models course written by expert Phil Ledgerwood.

Phil Ledgerwood has been a software developer for fifteen years. He currently works primarily in .NET technologies producing custom software for organizations of all sizes. He has also done extensive training for those same organizations in both technical and business process topics.