Moq and file/data saving in C#

Question

Ok, I am working on some unit testing, and my understanding is (based on "The Art of Unit Testing" by Roy Osherove) that if you're hitting a DB or actually saving a file, you're integration testing, not unit testing.

Given that, I have the following (yes, I know this violates SRP, but it illustrates my point):

public class PrimeChecker : IPrimeChecker
{
    public bool IsPrime(int num)
    {
        if (num < 2)
        {
            return false;
        }

        if (num > 2 && num % 2 == 0)
        {
            return false;
        }

        if (num % 2 != 0)
        {
            for (int i = 3; (i * i) <= num; i += 2)
            {
                if (num % i == 0)
                {
                    return false;
                }
            }
        }

        return true;
    }

    public bool Save()
    {
        return true;
    }
}

Note the the Save() method is simply a stub right now.

I'm trying to use Moq to simulate saving. So my test looks like this:

[Test]
    public void Save_WhenCalled_ExecutesSave()
    {
        var mock = new Mock<IPrimeChecker>();

        mock.Setup(x => x.Save());

        IPrimeChecker checker = mock.Object;
        checker.Save();

        mock.Verify(x => x.Save(), Times.Once);
    }

So if I had a fully-functioning Save() method in my class, is this the correct way to mock functionality? Because as I previously stated, my understanding is that for unit tests, I'm not supposed to actually write a file/save to a DB.

If this isn't correct, what should I be doing?

Answer 1

Assume your checker has a dependency on something needed to save.

Lets say

public interface IPrimeDb {
    bool Save(int prime);
}

And the checker looked like this

public class PrimeChecker {
    private IPrimeDb primeDb;

    public PrimeChecker(IPrimeDb db) {
        this.primeDb = db;
    }

    public bool IsPrime(int num) {
        if (num < 2) {
            return false;
        }

        if (num > 2 && num % 2 == 0) {
            return false;
        }

        if (num % 2 != 0) {
            for (int i = 3; (i * i) <= num; i += 2) {
                if (num % i == 0) {
                    return false;
                }
            }
        }

        return true;
    }

    public bool Save(int prime) {
        if (IsPrime(prime))
            return primeDb.Save(prime);

        return false;
    }
}

A test to test that the save is being called when a valid prime is provided can look like this.

[TestMethod]
public void Save_WhenCalled_ExecutesSave() {
    //Arrange
    var num = 3;
    var mock = new Mock<IPrimeDb>();

    var sut = new PrimeChecker(mock.Object);

    mock.Setup(_ => _.Save(num)).Returns(sut.IsPrime(num));

    //Act
    var actual = sut.Save(num);

    //Assert
    Assert.IsTrue(actual);
    mock.Verify(_ => _.Save(num), Times.AtMostOnce());
}

When unit testing you tend to mock the dependencies of the subject under test so as to avoid undesirable behavior. This allows the test to be exercised in isolation.