Unit tests with FluentValidation. How to mock ValidationResult - unit-testing

Question

I'm using TDD approach with xUnit 2, NSubstitute, AutoFixture, FluentAssertions for my unit tests.
I want test my service method which using FluentValidation.
Simple example:
Validator:
RuleSet("Nulls", () =>
{
RuleFor(viewModel => viewModel).NotNull();
});
My service(under test):
if(Validate(viewModel, "Nulls"))
//....
private bool Validate(AddMerchantViewModel viewModel, string option)
{
var result = _merchantValidator.Validate(viewModel, ruleSet: options);
return result.IsValid;
}
And my unit test:
I don't know how to mock the merchantValidator.Validate result.
[Theory, AutoNSubstituteData]
public void Add_ViewModelAsNull_ShouldThrowArgumentNullException(
AbstractValidator<AddMerchantViewModel> merchValidator,
MerchantsService service)
{
// Arrange
//here I don't know how to mock result of Validate. It is always null.
merchantValidator.Validate(Arg.Any<AddMerchantViewModel>(), ruleSet: Arg.Any<string>()).Return(new ValidationResult());
// Act
Action action = () => service.Add(null);
// Assert
action.ShouldThrow<ArgumentNullException>();
}

Answer 1

By default AutoFixture creates a new instance of a type every time it's requested. In this particular case the AbstractValidator<AddMerchantViewModel> type is instantiated twice - as the merchValidator parameter and as a dependency of the MerchantsService class.
As a result the configured validator is not used by the service. In order to fix that you should decorate the merchValidator parameter with the [Frozen] attribute so that the same instance of the AbstractValidator<AddMerchantViewModel> type is always returned by AF:
[Theory, AutoNSubstituteData]
public void Add_ViewModelAsNull_ShouldThrowArgumentNullException(
[Frozen]AbstractValidator<AddMerchantViewModel> merchValidator,
MerchantsService service)
// ...
More info abot the [Frozen] attribute can be found here.