With the release of Enterprise Library 3, a brand new Validation Block (Microsoft.Practices.EnterpriseLibrary.Validation) has emerged that allows developers to perform custom validation in an application based on custom validation setup for a business object. When the validation occurs, any validation errors are returned through a ValidationResult collection, so you can get a listing of all the problems that occurred with the validation of a business object.  However, with this new block comes integrated ASP.NET features. Custom validators (like the existing validation controls) can be used to perform the actual validation of the business object and the errors render in the ValidationSummary control. We will see how that works in the rest of the article.

Enterprise Library has several built-in validators that perform a certain validation of data. For instance, it has a StringLengthValidator object that validates that the length of a string falls within a certain range, or a DateTimeRangeValidator for validating that a date occurs within a specific range. You can instantiate these validators and perform validation or each validator has an attribute class associated with it so the validators can be defined as attributes to a property in a business object. This is how the ASP.NET integration can utilize the validation as we will see later. An alternative to this is that the configuration file can setup these validations, but I am going to use the attribute approach as I think it is cleaner and more understandable. Lastly, you can build your own custom validators, which is out of the scope of this article, but I hope to write about it later.

To integrate the business object validation into ASP.NET, the integration library contains a ParameterProxyValidator control that works like any other validator control in the ASP.NET framework. The validator has extra properties that map the validator to the business object property that you want to validate, assuming that the business object property maps directly to an ASP.NET control. Let us look at an example.