How to mock FluentValidation ValidationResults in test

Question

This is the method that I want to test:

public async Task<object> CreateExpense(CreateExpenseCommand command)
{
    var validationResults = await _validator.ValidateAsync(command);
    if (!validationResults.IsValid)
    {
        return validationResults.Errors.First().ToString();
    }
    //more code that is irrelevant for this post
}

To test this, I need to mock _validatior, which is defined as private readonly IValidator<CreateExpenseCommand> _validator; and is used through constructor injection.

I am using AutoFixture with AutoMoqCustomizations and Moq for mocking. Maybe I should use Moq exclusively?

This is what I try to do in the test:

[Fact]
public async Task CreateExpense_Success()
{
    //Arrange
    var service = _fixture.Fixture.Freeze<Mock<IValidator<CreateExpenseCommand>>>();
    service.Setup(x => x.Validate((CreateExpenseCommand)It.IsAny<IValidator<CreateExpenseCommand>>())).Returns(It.IsAny<ValidationResult>);
    
    //more code that is irrelevant for this post
}

However, this results in error:

System.NullReferenceException: 'Object reference not set to instance of an object'.

The error is pretty self-explanatory, but I don't know how to mock correctly.

Image of error:

Answer 1

I am very late to answer what solved it for me, but here goes:

namespace ExpenseTests.Application
{
    public class CreateExpenseServiceTests
    {
        private TestFixture _fixture;
        public CreateExpenseServiceTests()
        {
            _fixture = new TestFixture();
        }
        [Fact]
        public async Task CreateExpense_Success()
        {
            //Arrange
            var expense = _fixture.Fixture.Create<Expense>();
            CreateExpenseCommand command = _fixture.Fixture.Build<CreateExpenseCommand>()
                .With(c => c.ExpenseCategory, "HOTEL").Create();
            var validationResults = _fixture.Fixture.Create<ValidationResult>();

            var addMock = new Mock<ICreateExpense>();
            addMock.Setup(x => x.CreateExpense(command)).ReturnsAsync(expense);
            ICreateExpense addService = addMock.Object;

            var validatorMock = new Mock<IValidator<CreateExpenseCommand>>();
            validatorMock.Setup(x => x.Validate(It.IsAny<CreateExpenseCommand>())).Returns(validationResults);

            //Act
            var response = await addService.CreateExpense(command);

            //Assert
            Assert.Equal(typeof(Expense), response.GetType());
        }
    }
}

A lot of it is domain code that you can ignore. Let me know if you have further questions - hope it helps a bit.