Why mocking is good

As I said in my last post, unit testing is hard, and it’s something that can be hard to learn.

I’ve been fortunate on my current project to be working with guys who introduced me to mocking frameworks (in our case, Rhino Mocks). I had heard of Rhino Mocks before, but I never really looked into it. I thought, if I’m not testing against a database, how is that a valid test of what will actually happen in the application?

I can think of a very simple example that I think will show the value of mocks. Let’s say that it’s your first day on a brand new, green field project where you’re writing a system that deals with claims. You create a simple Claim object and you want to write a unit test to make sure that your validation code checks to see if the ClaimNumber property has a value. Your tests (not using mocks) might look like this:

[TestMethod]
public void SaveClaimPositiveTest()
{
    ClaimLogic logic = new ClaimLogic();
    Claim c = new Claim();
    c.ClaimNumber = "12345";
    logic.Save(c);
    
    Claim loadedClaim = logic.Get(c.Id);
    Assert.IsTrue(loadedClaim != null);
    Assert.IsTrue(loadedClaim.ClaimNumber == c.ClaimNumber);
}

[TestMethod]
public void SaveClaimNegativeTest()
{
    ClaimLogic logic = new ClaimLogic();
    Claim c = new Claim();
    c.ClaimNumber = null;
    try
    {
        logic.Save(c);
        Assert.Fail("Expected exception was not thrown.");
    }
    catch (MyValidationException ex)
    {
        Assert.IsTrue(ex.Property == "ClaimNumber", "Validation for ClaimNumber did not occur.");
    }
}

These are fairly simple tests that I’ve written many times in the past. What is wrong with these tests? Nothing right now. But that is all going to change.

What happens when someone adds a new property to the Claim object and adds a validation rule that says that the new field is required? My tests break. Now I have to go back and add more code to the beginning to create a valid Claim object that I can save.

What happens when someone adds some more logic to the Save method that kicks off some complicated workflow? Now my simple tests are getting a lot more complicated, and I’m going to have to deal with all kinds of side effects and errors just to test a really simple validation rule.

I’m also cluttering up the database with test data. So I need to write some cleanup code to delete the Claim object I inserted. Later on someone will add some foreign key relationships to the table, and I’ll have to come back add more code to clean up those related objects too. Someone might add some triggers too, and I’ll have to account for that. Those triggers might make it hard (or impossible) to clean up this test data.

These tests are also going to be pretty slow because they have to interact with the database. Yes, it’s a simple test, but what happens when I have 2000 of these tests? It’s going to take a long time to run them all!

All I wanted to do was verify whether my validation code was written correctly! Why do I have to actually try and save the object to the database to do this?

What happened on past projects where I wrote these kinds of tests was this: the unit tests take 5 minutes to run, so no one runs them when they check in. Eventually someone does something that breaks the test (usually harmless stuff like adding validation rules), but no one takes time to fix it. Before you know it, half of the unit tests are broken, but it’s getting close to your release date, so you don’t have time to fix them. Then, sometime after the release, someone will spend a week updating the tests so that they all work again.

This is where mocking comes in.

I’m not going to go in depth into mocking because there are plenty of people who have written in depth posts on how to do this. But the basic idea is that I am going to “mock out” external dependencies by telling the mocking framework what those external dependencies are going to do and return. In this case, I’ll mock out the code that would actually do the actual saving to the database because all I’m testing is the validation.

Yesterday I wrote a unit test for a method that returned a list of users in the application who were in certain security roles, and the list of users returned depended on the current user’s security role (if you’re in certain roles, I don’t want the list of users to include people in certain other roles). The list of users is stored in a database table, and the security roles are stored in Active Directory groups.

In this case, it would be pretty much impossible to test this without mocks. Think about what I have to do:

1) Test that the list of users returned will be correct depending on the user’s security role
2) Test that users in each security role will be returned at the proper time

The problem is that I don’t know what users are in the database and I don’t know what users are in each role in Active Directory. I don’t have a way to write code to insert users into Active Directory groups. Even if I could do that, I would then have to write code to insert a bunch of test users into the database (or even worse, write a test that would expect certain actual users to be in the database). Then I’d have to clean everything up when the test finished.

I don’t need a database or Active Directory to test my logic that would return the correct list of users. I can mock out the database by just using a list of User objects instead, and I can mock out Active Directory by just telling the mock framework what users to return for each role.

I wrote 16 tests around this section of code and they all run in about 5 seconds! I don’t have to insert any test code anywhere, I don’t have to clean it up, I don’t have to worry about people adding new validation rules, and I don’t have to worry about people adding other external code that will break my test (that wouldn’t otherwise have an effect on the code I was testing).

I will say this — mocking is not easy. It really is an art. It is not something that I learned overnight. Even though I understood the concept, I didn’t really get good at it until recently. But now that I understand it, I write much better unit tests, I write more unit tests, and I can do it pretty quickly. Test Driven Development (writing tests first) is also an art, and it’s something that I’m just starting to get into.

When I first started programming, I didn’t care about unit tests because I just wanted my programs to work. Sadly, in my four years of college, no one even mentioned a unit test. I didn’t even know what one was when I started my first job! Trying to reprogram yourself to work in a TDD way is hard because you’re breaking old habits that you’ve had since the day you first started programming.

Mocking and TDD will revolutionize the way that you write unit tests, but like I said, it can feel like you’re learning a foreign language. If you have someone that you know that is good at this stuff, beg them, plead with them, take them out to lunch, or do whatever else you have to do to get them to sit with you and pair program for a day or two and learn from them. You won’t want to go back!

Also, if you don’t have a continuous integration build that runs all of your unit tests every time you check in, get one. We’re using TFS 2008 and I created a CI build using TFS in less than 5 minutes using the TFS build definition wizard. Then make sure that you fix broken tests as soon as they break!

Happy mocking!