Designing Custom Validation Attributes

A custom validation attribute can be implemented by inheriting from the ValidationAttribute class. Server-side validation code is provided by overriding the methods that perform validation and error message formatting.

Client-side validation requires that our attribute implements the IClientModelValidator interface whose unique AddValidation method specifies the data- attributes to add to the input field. The actual client-side validation code must be provided through a JavaScript validator which is connected to the parser that processes all data- attributes through a JavaScript adapter.

We will describe all the above steps while implementing an example custom validation attribute: the ExtensionAttribute.

The ExtensionAttribute

As an example, we will define an attribute that checks the extension of a Web Address. It is useful when combined with the UrlAttribute to force the user to insert the address of a pdf, or of a supported image type. The basic idea is to pass it a string with all comma-separated allowed extensions. Something like .jpg,.jpeg, or .png. The usage should be similar to the example below: