In every release of Catel, we try to improve a significant part of the code based on user requests. For release 2.4, it was the turn of the validation.

The validation in Catel is extremely flexible, and allows to use validation in the model (exposed via INotifyDataErrorInfo or IDataErrorInfo), validation in the view model (via the Validate methods) or via attributes (data annotations) on either the model or view model. However, this doesn’t seem enough for some users that require the absolute freedom in validation. Therefore, after intense discussions with the users, we came up with the following changes.

Introduction of IValidator and IValidatorProvider

The first request was to allow external validators to get involved into the validation process. For example, some use Fluent Validation for advanced validation checks. In previous versions of Catel, the calls to the external validators had to be implemented manually and converted to IValidationResult interfaces.

To make it much easier to hook into the validation process, the IValidatorProvider and IValidator were developed. The IValidatorProvider is a class that returns the right IValidator for a specific type. If no validator is available, it will return null. The ViewModelBase will automatically check the ServiceLocator whether an IValidatorProvider is available, and will then retrieve the IValidator for the current instance automatically.

The IValidator itself contains lots of methods, and allows to hook into the following events:

• BeforeValidation
• BeforeFieldValidation
• FieldValidation
• AfterFieldValidation
• BeforeBusinessRuleValidation
• BusinessRuleValidation
• AfterBusinessRuleValidation
• AfterValidation

For example, it can use Fluent Validation in the FieldValidation  and BusinessRuleValidation, and can translate the errors in AfterValidation.

Introduction of IValidationContext

Another nice improvement is the creation of the IValidationContext. It takes over the responsibility as validation container from the DataObjectBase. This way, the DataObjectBase becomes a bit less complicated and at the same time, the new container allows querying of all validations of a specific object in a very flexible way. Below are a few examples on how to query validations of a specific object:

First, the ValidationContext can be retrieved from a view model like this:

   1: var viewModel = new MyViewModel();

   2: var validationContext = viewModel.ValidationContext;

Get all field errors with a specific tag

   1: var fieldErrorsWithTag = validationContext.GetFieldErrors((object)"myTag");

Get all field validations of a specific property

   1: var fieldValidationResultsForProperty = validationContext.GetFieldValidationResults("MyProperty");

Get all business rule warnings

   1: var businessRuleWarnings = validationContext.GetBusinessRuleWarnings();

We hope to release Catel 2.4 this weekend (waiting on the last feedback), so you don’t have to wait long for these features .