ASP.NET MVC 2: Model Validation
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by Scott Guthrie
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Step 2: Enabling Validation using DataAnnotations

Let’s now update our application to enforce some basic input validation rules.  We’ll implement these rules on our Person model object – and not within our Controller or our View.  The benefit of implementing the rules within our Person object is that this will ensure that the validation will be enforced via any scenario within our application that uses the Person object (for example: if we later added an edit scenario).  This will help ensure that we keep our code DRY and avoid repeating rules in multiple places.

ASP.NET MVC 2 enables developers to easily add declarative validation attributes to model or viewmodel classes, and then have those validation rules automatically be enforced whenever ASP.NET MVC performs model binding operations within an application.  To see this in action, let’s update our Person class to have a few validation attributes on it.  To do this we’ll add a “using” statement for the “System.ComponentModel.DataAnnotations” namespace to the top of the file – and then decorate the Person properties with [Required], [StringLength], [Range], and [RegularExpression] validation attributes (which are all implemented within that namespace):

Note: Above we are explicitly specifying error messages as strings. Alternatively you can define them within resource files and optionally localize them depending on the language/culture of the incoming user.  You can learn more about how to localize validation error messages here.

Now that we’ve added the validation attributes to our Person class, let’s re-run our application and see what happens when we enter bogus values and post them back to the server:

Notice above how our application now has a decent error experience.  The text elements with the invalid input are highlighted in red, and the validation error messages we specified are displayed to the end user about them.  The form is also preserving the input data the user originally entered – so that they don't have to refill anything.  How though, you might ask, did this happen? 

To understand this behavior, let’s look at the Create action method that handles the POST scenario for our form:

When our HTML form is posted back to the server, the above method will be called.  Because the action method accepts a “Person” object as a parameter, ASP.NET MVC will create a Person object and automatically map the incoming form input values to it.  As part of this process, it will also check to see whether the DataAnnotation validation attributes for the Person object are valid.  If everything is valid, then the ModelState.IsValid check within our code will return true – in which case we will (eventually) save the Person to a database and then redirect back to the home-page. 

If there are any validation errors on the Person object, though, our action method redisplays the form with the invalid Person.  This is done via the last line of code in the code snippet above.

The error messages are then displayed within our view because our Create form has <%= Html.ValidationMessageFor() %> helper method calls next to each <%= Html.TextBoxFor() %> helper.  The Html.ValidationMessageFor() helper will output the appropriate error message for any invalid model property passed to the view:


The nice thing about this pattern/approach is that it is pretty easy to setup – and it then allows us to easily add or change validation rules on our Person class without having to change any code within our controllers or views.  This ability to specify the validation rules one place and have it be honored and respected everywhere allows us to rapidly evolve our application and rules with a minimum amount of effort and keep our code very DRY.

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Title: asa   
Name: sd
Date: 2012-11-12 9:15:33 AM

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