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Derek Ferguson on "Pragmatic Unit Testing" An Introduction to NMock

I differentiate what I like to call 'pragmatic unit testing' from the two alternate approaches

I have staunchly advocated what I like to call "pragmatic unit testing" at every client I have been at for the past year. I differentiate pragmatic unit testing from the two alternate approaches to unit testing that I have seen at organizations. The first "alternate approach," which I have seen at 95 percent of .NET organizations, is best referred to as "no time for quality." The other approach, which I have mainly seen at J2EE organizations, can be labeled "quality, even if it puts us out of business."

Between these two extremes, I have tried to follow a path where I use unit tests to accomplish my daily work. I try to build as much resilience and reusability into these unit tests only as my immediate deadlines allow. This means that after a project is finished, I often have a decent-sized set of unit tests that have been created without costing my client anything in terms of development time.

The downside to this approach is: because I haven't spent any extra time in abstracting these unit tests away from their dependencies (since those dependencies happened to exist at precisely the moment of their creation), half of my unit tests typically wind up broken shortly after release, through no cause other than the application's standard usage.

Let me clarify this rather difficult-to-explain but easy-to-see phenomenon with a simple example. I have spent the last couple of months creating a fairly straightforward Windows application that is intended to store and retrieve data from a database, as well as perform some calculations on that data using Visual Studio Tools for Office and an Excel spreadsheet back end. For unit testing the calculations and the Excel back end, I have simply hard-coded specific data into my unit tests, which have exercised all of the spreadsheet's various functions. This has worked well and my unit tests are serving their purpose wonderfully.

For the database interactions, however, I have faced some difficult issues in creating appropriate unit tests. On one hand I have a frequently changing data model. This breaks my unit tests a lot but, on the whole, this is fine because I would specifically like my unit tests to alert me to breaking changes in the database structure.

On the other hand, many of my unit tests are not specifically testing database interactions, but instead test functionality, which is so complex that it is highly dependent upon the data that happens to be in the database and its structure. As a result, when the data in the database changes, my unit tests - which are expecting results that are specific to the state of the data when they were written - break. I then have to go in, examine the failed assertions, and change them to match the new data in the database. Eventually, the data changes again and they fail again.

Enter Mock Objects
I have been familiar with mock objects for several years. I've realized that they could provide solutions to my problems. However, at my former employer, there was a developer who used to spend hours, days, and even weeks creating elaborate mock infrastructures to have perfect unit tests - often at the cost of actually shipping software on time. Having seen this I, of course, was none-too-enthusiastic about mock objects.

Then, a colleague at Magenic posted to our internal technical lists about NMock. NMock, it seemed, held the potential to allow me to create mock objects to solve my problems without having to spend lots of time in their creation. "Cool," thought I.

Thus, the first thing to understand about NMock is that it is free. Well, the license says that you have to keep the copyright notice, but it looks to me (though I am not a lawyer) as though you can redistribute it and use it in various other products, including commercial ones, so I would call that free.

The second thing to understand is that NMock creates mock objects at run time, rather than doing pre-generation. This went against the way I had envisioned a mock object generator would work before I had ever seen one, so I was a little surprised by this. The creators say that this makes mock objects created with NMock more resilient and avoids a pre-usage compilation step. I guess I can see that.

Taking It for a Test Drive
Therefore, for my first crack at using mock objects, I decided to choose one of my existing unit tests that had been breaking. The test had to do with authorization of users into my system. It was intended to find a user in the database, retrieve a number of facts about that user, and then assert that those facts matched the facts of which I have a priori knowledge.


[TestFixture]
public class UserTest
{
[Test]
public void VerifyRoleRetrieval()
{
User user = new User();
user.Authorize("derek");

Assert.AreEqual(5, user.Roles.Count);
}
}
As you see in the above test, I begin by creating a new User object. I then call "Authorize" on the object, which causes it to populate itself with data about the requested user (in this case, myself). Finally, I assert that the number of roles associated with my own user account is five, because I know in advance that that is how many roles will be on the user object, if the population has been correctly done.

Unfortunately, as you may have guessed, the number of roles associated with my account on our system varied continually during development, which made this test difficult to maintain, at best. Once the system went into production it wasn't quite as bad, but it still wasn't ideal.


public class User
{
ArrayList roles;

public void Authorize()
{
ArrayList roles = new ArrayList();
// IP for populating roles removed here
}

public ArrayList Roles
{
get
{
return roles;
}
}
}
The code above shows - more or less - what happens inside of the User class to make the authorization and role-recollection functionality work. I had to remove some code from the Authorize method to protect my client's Intellectual Property but, nevertheless, you can see that a simple ArrayList is used to contain the roles. The code I removed was tied too closely to an actual database, thus making it difficult to unit test.

Following the approach documented right on NMock's home page, I adapted my user class to take a reference to an "Authorizer" class. The Authorizer class would contain my DB logic in the real application, but in my unit tests, it is what will be mocked. For this reason I have declared it as an interface in Listing 1, which different classes can implement depending upon the circumstances (test or production).

The code in Listing 2 shows my first cut at amending my tests to use an automatically generated unit test. This compiled and ran but, unfortunately, it did not pass since there is nothing to tell NMock to actually return an ArrayList with five members as a part of this test.

Listing 3 shows the modifications I had to make in order to finally build a good unit test for my User class's Authorization functionality. In my unit test I had to create an artificial unit test for my User class to return. I then had to change my call to the "Expect" method on the dynamically generated mock to be "ExpectAndReturn," passing in the value for it to return, in addition to my expectations about the way that the Authorize method should be called.

The Authorize method populates the internal roles variable in the User class so that a little Assert test that expects five members will succeed.

Conclusion
My conclusion after using NMock for about an hour is that it is definitely a good way to help create unit tests that will serve well beyond the initial creation of your code. My other realization was that in some ways, seeing that your code requires re-architecture in order to support easy mocking can be an indication that one hasn't architected one's initial code with the greatest possible degree of flexibility, to begin with.

I anticipate adding NMock to my bag of tricks and using it extensively going forward. To read about my further experiences with unit testing, please read my blog at http://derek.dotnetdevelopersjournal.com/.

More Stories By Derek Ferguson

Derek Ferguson, founding editor and editor-in-chief of .Net Developer's Journal, is a noted technology expert and former Microsoft MVP.

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