Validation is an important part of any data-driven web application. Client-Side validation provides fast user feedback and a better UI experience but cannot be relied on for data integrity - so client-side validation should always be backed by additional server-side validation.

With MVC Unobtrusive Validation, you can configure both client-side and server-side in one place. 

Validation rules can be added to a model object via Data Annotations or using the Fluent Validation API.

Fluent Validation is available as a Nuget package. See Do you use Fluent Validation?

Figure: OK Example - Data Annotation attributes decorate model properties to make them required

Figure: Better Example - Fluent Validation allows validation metadata to be added to a class without modifying the original class.  This provides much more flexibility for code reuse

If you create a new MVC web application in VisualStudio 2013, unobtrusive validation will be enabled by default. Otherwise, it's simple to install from Nuget. To use it simply:

1. ​Bind your razor views to model objects 
   
2. Use Html Helpers to render the form UI

Figure: Good Example - this razor view binds to a strongly typed model object and uses HTML helpers.

Figure: the HTML UI rendered for this view now has data-validation attributes that are followed by JQuery validation to provide rich client-side validation.

Figure: On the server-side, the same validation rules will be checked when you call ModelState.IsValid