The validation framework of ASP.NET MVC is designed in such a way that it could be easily customizable/extensible at many points. The validation system is built using lot of classes and it's quite difficult to understand all of them. Sadly, there is no much documentation available in MSDN to help out. As default, the validations are performed by decorating models and properties with validation attributes. These validation attributes are available in a separate assembly called System.ComponentModel.DataAnnotations.

Sometimes, in applications we need to apply or perform validations in different ways. For ex. say we want to store the validation rules for a model in database or in xml files. In these cases, we have to go for implementing custom validation solutions and for that understanding the built-in validation model is trivial.

In this article, we are going to see how we can create a custom ModelValidatorProvider that validates models based upon the rules specified in the xml files and works side-by-side with the other built-in validator providers.

In a web application the domain classes and the validations associated with those classes forms the Model. Validation plays a core part in a Model. In ASP.NET MVC, model validations are done by using Data Annotations. Data Annotations are nothing but special attributes that are applied to a class or properties of a class. In many cases these built-in validation attributes are not sufficient to fulfill our business requirements and in those cases we can go for building our own custom validations.

In this article we will see how to apply basic validations to a model and also we will see how to create custom validations by implementing the ValidationAttribute class or IValidatableObject interface.