The validation in Catel is extremely flexible, at this point you must already know it, but sometimes it is just not enough or you are forced to use external validators.

FluentValidation is a small validation library for .NET that uses a fluent interface and lambda expressions for building validation rules for your business objects. Catel provides an extension in order to use FluentValidation as a validation library.

The only thing you have to do is install an isolate package named Catel.Extensions.FluentValidation, available via NuGet, then you will be able to write your view models validations using FluentValidation approach.

public class PersonViewModelValidator : AbstractValidator<PersonViewModel>
{
    public PersonViewModelValidator()
    {
        RuleFor(person => person.FirstName).NotEmpty();
        RuleFor(person => person.LastName).NotEmpty().WithMessage("Please specify the last name");
    }
}

In order to retrieve the right validators, you must register the FluentValidatorProvider:

ServiceLocator.Default.RegisterType<IValidatorProvider, FluentValidatorProvider>();

The FluentValidatorProvider will automatically retrieve the right validators associated with the view models.

How handle all Catel validation concepts with fluent validation classes?

Catel handle concepts like field or business rules errors and warnings . So, it's necessary map the fluent validation class to the specific Catel validation using ValidationDescriptionAttribute.

[ValidationDescription(Tag = "Person", ValidationResultType = ValidationResultType.Error, ValidationType = ValidationType.Field)]
public class PersonViewModelValidator : AbstractValidator<PersonViewModel>
{
}

How FluentValidationProvider works?

FluentValidationProvider is an implementation of IValidationProvider (see: Validation via IValidator). It search all validators classes that implements FluentValidation.IValidator interface, that also can validate the view model type.

A view model can have one or more validators, so FluentValidationProvider aggregates all validators in a single one using CompositeValidator class. For performance reasons FluentValidationProvider only searches for validators on the assembly which the view model belongs to.