Test Extensions

FluentValidation provides some extensions that can aid with testing your validator classes.

We recommend treating validators as ‘black boxes’ - provide input to them and then assert whether the validation results are correct or incorrect.

Simple test extensions

FluentValidation comes with two extension methods ShouldHaveValidationErrorFor and ShouldNotHaveValidationErrorFor that can make it easier to write unit tests for validators. For example, imagine the following validator is defined:

public class PersonValidator : AbstractValidator<Person> {
   public PersonValidator() {
      RuleFor(person => person.Name).NotNull();

You could ensure that this validator works correctly by writing the following tests (using NUnit):

using NUnit.Framework;
using FluentValidation;
using FluentValidation.TestHelper;

public class PersonValidatorTester {
    private PersonValidator validator;

    public void Setup() {
       validator = new PersonValidator();

    public void Should_have_error_when_Name_is_null() {
         validator.ShouldHaveValidationErrorFor(person => person.Name, null as string); 
    public void Should_not_have_error_when_name_is_specified() {
        validator.ShouldNotHaveValidationErrorFor(person => person.Name, "Jeremy");

If the assertion fails, then a ValidationTestException will be thrown.

Note that if you have a complex validator that relies on multiple properties being set, you can pass in a pre-populated instance rather than just the property value:

    public void Should_not_have_error_when_name_is_specified() {
        var person = new Person { Name = "Jeremy" };
        validator.ShouldNotHaveValidationErrorFor(x => x.Name, person);

Advanced test extensions

The methods ShouldHaveValidationErrorFor and ShouldNotHaveValidationErrorFor used in the examples above are wrappers around FluentValidation’s TestValidate functionality. If you have more complex tests, you can use TestValidate instead to perform multiple assertions on a single validation result. For example:

var person = new Person { Name = "Jeremy" };
var result = validator.TestValidate(person);

// Assert that there should be a failure for the Name property.
result.ShouldHaveValidationErrorFor(x => x.Name);

// Assert that there are no failures for the age property. 
result.ShouldNotHaveValidationErrorFor(x => x.Age);

// You can also use a string name for properties that can't be easily represented with a lambda, eg:

You can also chain additional method calls to the result of ShouldHaveValidationErrorFor that test individual components of the validation failure including the error message, severity error code and custom state:

var result = validator.TestValidate(person);

result.ShouldHaveValidationErrorFor(x => x.Name)
  .WithErrorMessage("'Name' must not be empty.")

There are also inverse methods avaialble (WithoutMessage, WithoutErrorCode, WithoutSeverity, WithoutCustomState)