Cannot Mock<IValidator<Entity>> and ThrowException

Which version of FluentValidation are you using? 9.1.1

Describe the issue that you’re having On previous version I can Mock<IValidator<T>> setup Validate returning ValidationResult with List<ValidationFailure> and test for throwing ValidationError

var validator = new Mock<IValidator<Entity>>();
validator.SetupFail();

// Validator is passed to another class that call 
// validator.Validate(instance, options => options.ThrowOnFailures());

await Assert.ThrowsAsync<ValidationException>(() => handler.Handle(request, validator.object));

// Extension method

public static ValidationResult FailResult => new ValidationResult(new List<ValidationFailure>() { new ValidationFailure("Foo", "Bar") });

public static Mock<IValidator<T>> SetupFail<T>(this Mock<IValidator<T>> validator)
{
	
	validator.Setup(p => p.Validate(It.IsAny<T>())).Returns(FailResult).Verifiable();
	validator.Setup(p => p.ValidateAsync(It.IsAny<T>(), It.IsAny<CancellationToken>())).ReturnsAsync(FailResult);

	validator.Setup(p => p.Validate(It.IsAny<IValidationContext>())).Returns(FailResult).Verifiable();
	validator.Setup(p => p.ValidateAsync(It.IsAny<IValidationContext>(), It.IsAny<CancellationToken>())).ReturnsAsync(FailResult);
	
	return validator;
}

Expected Behaviour Throw Exception

Actual Behaviour No Exception is Throw

Note I looked on source code and I found the code below on AbstractValidator; question: is still possible to mock IValidator for throwing exception when error ?

protected virtual void RaiseValidationException(ValidationContext<T> context, ValidationResult result) {
	throw new ValidationException(result.Errors);
}

A possible solution is to add to IValidationContext and set public internal bool ThrowOnFailures { get; set; }

When setup mocking I can select to return errors or raise error.

[JeremySkinner]

I’ve pushed out 9.1.2, which makes the getter public on ValidationContext.

[JeremySkinner]

in some situation throwing exception by validator is a trigger for other code.

Yes, which is why it’s better to have a real validator instance which receives valid/invalid input to trigger a “real” exception. If you mock this, then you can end up in a situation where the mock throws an exception, but the real validator wouldn’t (eg if the RaiseValidationException method was overridden, or some other change had been made). By mocking the validator you’re assuming it’ll behave in a particular way when given certain input, but that isn’t necessarily the case, which leads to brittle tests and a false safety net.

I will make the getter of that property public on ValidationContext<T> (but not the interface), so you can do this if you want to, but I would still highly recommend you take a black-box approach instead. I’ll try and get this done tomorrow.