We’ve built a nice, simple data-entry application.

One problem with it, though, is that we don’t currently have any type of input validation in place to ensure that fields are filled out correctly within our Create Dinner form.  Let’s fix that.

Adding Validation using DataAnnotations

Validation rules in an ASP.NET MVC based application are usually best expressed within a model.  This enables them to be maintained in a single place, and enforced across any number of controllers and views that might interact with them.  ASP.NET MVC enables you to implement validation rules using a variety of different mechanisms, and is flexible enough to support just about any validation scheme you want to use. 

ASP.NET MVC 2 includes built-in support for using .NET’s System.ComponentModel.DataAnnotations library of validation rules – which enable you to declaratively apply validation rules to model classes using validation attributes.  You can learn more about this capability in a previous blog post I wrote.  We’ll take advantage of this approach to enable input validation for our NerdDinner application.

Let’s go back to the Dinner class we defined earlier and add some validation attributes to its properties (note: we need to add a “using System.ComponentModel.DataAnnotations” namespace as well):

The [Required] validation attribute indicates that a particular property must be specified.  The [StringLength] validation attribute allows us to indicate a maximum length for a particular string property.  The [RegularExpression] validation attribute allows us to indicate that a particular string property must match a specified regular expression in order to be valid – in this case an email address.

Each of the validation attributes supports an “ErrorMessage” property – which allows us to specify an error message that should be displayed if the validation fails.  This can either be hard-coded as a string (like above) or pulled from a resource – enabling it to be easily localized.