Code Coverage Teaches and Protects
Posted by bsstahl on 2016-10-14 and Filed Under: development
I often hail code coverage as a great tool to help improve your code base. Today, my use of Code Coverage taught me something about the new .NET Core tooling, and helped protect me from having to support useless code for the lifespan of my project.
In the code below, I used a common dependency injection pattern. That is, an IServiceProvider object holding my dependencies is passed-in to my object and stored as a member variable. When a dependency is needed, I retrieve that dependency from the service provider, and then take action on it. Since there is no guarantee that the dependency I need will have been placed in the container, I use some common guard logic to protect my code.
templates = _serviceProvider.GetService<IEnumerable<Template>>();
if ((templates==null) || (!templates.Any(s => s.TemplateType==ContactPage)))
throw new TemplateNotFoundException(TemplateType.ContactPage, string.Empty);
In this code, I first test that I was able to retrieve a collection of Template objects from the service provider, then verify that the type of Template I need is present in the collection. If either is not the case, an exception is thrown.
I had two tests that covered this section of code, one where the collection was not added to the service provider, the other where an empty collection was added. Both tests passed, however, it wasn't until I looked at the results of the Code Coverage that I realized that the 1st test wasn't doing what I thought it was doing. It turns out that there is actually no way to get a null collection object out of the Microsoft.Extensions.DependencyInjection.ServiceProvider object I am using for my .NET Core apps. That provider simply returns an empty collection if there isn't one in the container. Thus, my check for null was never matched and that branch of code was never executed.
Based on this new knowledge of the behavior of the IServiceProvider, I had a few options. I could:
- Rewrite my test to check for an empty collection. This option seems redundant to me since my check to see if the container holds the template I need is really what I care about.
- Leave the code as-is just in case the behavior of the container changes, accepting that I have what is currently unnecessary and untestable code in my application. I considered this option but it seems to me that a better defense against the unlikely event of a breaking change in the IServiceProvider implementation is described below in option 3.
- Create a new test that verifies the behavior on the ServiceProvider that an empty collection is returned if no collection is supplied to the container. I am not a big fan of this option since it requires me to test OPC (other people's code), and because the risk of this type of breaking change is, in my opinion, extremely low.
- Remove the guard code that tests for null and the test that supports it. Since the code is completely unnecessary, the test itself is redundant because it is, essentially identical to the test verifying that the template I need is in the collection.
I'm sure you've guessed by now that I selected option 4. I removed the guard code and the test from my solution. In doing so, I removed dead code that served no purpose, but would have to be supported through the life of the project.
For those who might be thinking something similar to, "It's nice that the coverage tooling helped you learn about your code, but using Code Coverage as a metric is actually a bad idea so I won't use Code Coverage at all", I'd like to remind you that any tool, such as a hammer or a car, can be abused. That doesn't mean we don't continue to use them, we just make certain that we use them properly. Code Coverage is a horrible way to measure a development team or effort, but it is an outstanding tool and should be used by the development team whenever possible to discover things about the code base.
TDD Helps Validate Your Tests
Posted by bsstahl on 2016-03-05 and Filed Under: development
One of the reasons to use TDD over test-later approaches is that you get a better validation of your tests.
When the first thing you do with a test or series of tests is to run them against code that does nothing but throw a NotImplementedException, you know exactly what to expect. That is, all tests should fail because the code under test threw a NotImplementedException. After that, you can take iterative steps to implement the code. Along the way, you should always see your tests fail in appropriate ways. Eventually, all of your tests should pass when the code is complete.
If tests start passing before they should, continue to fail when they shouldn’t, or fail for reasons that are different than what you’d expect at that point in the development process, you have a good indication that the test may not be doing what you want it to be doing. This additional information about the tests can be very helpful in making sure your unit tests are properly testing your code.
Think about what happens when you add tests after the code has already been written. Suppose you write a test for existing code, and it passes. What do you really know about the test? Is it working because it is adequately exercising your code? Did you forget to do an assert? Is it even testing the proper bit of code? Code coverage tools can help with some of this but they can only help if the code under test is not already touched by other tests. Stepping through the code in debug mode is another possibility, a third option is to comment out the code as if you were starting from scratch, effectively doing a TDD process without any of the other benefits of TDD.
What about when you write a test for previously written code, and the test fails? At this point, there are 2 possibilities:
- The code-under-test is broken
- The test is broken
You now have 2 variables in the equation, the code and the test, when you could have had only 1. To eliminate 1 of the variables, you have to again perform the TDD process without most of its benefits by commenting out the code and starting from ground zero.
Following a good TDD process is the best way to be confident that any test failures indicate problems in the code being tested, instead of the tests themselves.
Remove Any Code Your Users Don't Care About
Posted by bsstahl on 2015-09-28 and Filed Under: development
Code Coverage has been the topic of a number of conversations lately, most recently after the last Southeast Valley .NET User Group meeting where Jeremy Clark presented his great talk, Unit Testing Makes Me Faster. During this presentation, Jeremy eponymized, on my behalf, something I've been saying for a while, that the part of an application that you don't need to test is the part that your users don't care about. That is, if your users care about something in your application, you should be writing tests that ensure that the users' needs are fulfilled by your code. This has never really been a controversial statement, just one that sometimes gets lost in the myriad of information about unit testing and test driven development.
Where the conversation got really interesting was when we started discussing what should happen if you decide that a piece of code really isn't important to your users. It is my assertion that code which is deemed unimportant enough to the user that it might not be tested, should be removed from the project, even if is part of a standard implementation. I will attempt to justify this assertion by using the example of a property implementation that supports the INotifyPropertyChanged interface.
A visualization of the results of Code Coverage analysis on a typical property implementation. The blue highlights represent code that is covered by tests, the red highlights represent code that is NOT covered by tests.
In this example, we have a property getter and setter. The getter simply returns the value stored in the internal member. However the setter holds some actual logic. In this case, the new value being set is compared to the current value of the property. If the property value is changing, the update is made and a method called that fires a notification event indicating that the value of the property has changed. This is a fairly common implementation, especially for View-Model layer code.
Decision: Do my users care about this feature?
The conditional in this code is designed to skip the assignment and the change notification if the property value is not really changing. If we were to eliminate the conditional, it would impact the users of this code in the following ways?
- A few CPU cycles may be wasted on an assignment that isn't doing anything
- An event indicating the property was changed would fire incorrectly
In the vast majority of cases, the performance hit from item 1 is trivial and can be ignored. Item 2 however is a bit more complicated. Unless I know for certain that firing the event when the property is not really changing isn't a problem, I have to assume it is a problem, since there are any number of things that could happen as a result of having an event fire. Often, when this event fires it will cause a refresh of the bound data to the UI elements. This may have a significant impact on performance, or it may not. There may also be additional actions taken by the programmers of this event client that may not be foreseeable when designing this layer. If the circumstances are such that I know there will be no problems if the event fires more often than it should, then I can probably conclude that my users don't care about this code. In all other circumstances, I should probably conclude that they do.
Decision: Should I remove this code?
If I have concluded that my users care about the code, then my path is clear, I should leave the code in place and write tests to make sure that the event fires when it should, and only when it should. However, if I have concluded that my users don't care about this particular code, then I have another decision to make. I need to decide if I should leave the code untested but in place, remove the code from my project, or leave it in and write tests for it anyway.
If the feature is not important to the users and there is no likelihood that the feature will become important to the users in the future, then the code should not be there. Period. We cannot waste time and effort supporting code that our users will not need. Scope-creep is a real danger to any project and should be avoided at all costs, even on the small stuff. Lots of small stuff adds up to big stuff, especially over the lifespan of any non-trivial application.
So, if the features are important to the users, we test them, if they are unimportant to the users, we remove them. No controversy here. The questions come in when there is a likelihood that the feature could become important in the future, or if the feature is important to someone other than the users, such as the developers.
Suppose we decide that the users are likely to request this feature in the future. Wouldn't it be easier just to implement the feature now, when we are already in the code and familiar with it? My answer to this is to fall back on YAGNI. You Ain't Gonna Need It, has proven itself a valuable principal for preventing scope-creep. Even if you think it is pretty likely that you'll need something later, the reality is that you probably won’t. Based on this principal, we should not be putting features into our projects that are not needed right now.
But what about the situation where code is important to someone other than the users, for example, the developers? In this case, we have to decide if the code really is important, or is it just another case where the YAGNI principal should be applied. Technical requirements can be legitimate, but any requirement that is not directly in support of the user's needs is a smell that should be investigated. In the case of our property setter, saying that standardization is important and using that logic to make standardization a requirement sounds a lot like saying "I think this feature may be important someday" and it probably falls to YAGNI to keep it out of our code. That being said, if there is a technical requirement that is truly needed, it should be tested like any other important requirement. For a little more information on this, see my earlier analysis Conflict of Interest: Yagni vs. Standardization.
How About we Leave It and Just Don't Test It?
It is important to remember that we shouldn't simply leave code untested in our production code, even if the users don't really care about it right now. If we do so, and the feature becomes important in the future, we will almost certainly end up with code that is important to our users, but is untested and therefore at-risk. We are unlikely to go back into an application and just add tests for a feature that already exists simply because that feature is now important when it wasn't earlier. We'd like to think we would, but the fact is that we won't. No, leaving the code in the application, but untested, is not an option.
The Case for 100% Code Coverage
So, we want to remove any code that is not currently required by our users, and test all code that is truly needed. If you have come along with me on this you may now realize that 100% code coverage is actually a reasonable goal, since that would be the result of removing all unneeded code and testing all needed code. This is not to say that it is reasonable to use Code Coverage as a metric with which to judge a development team, but instead it should be considered as a tool that can help identify scope-creep and missing tests. Since we are testing all code that our users care about, and not adding any code that the users don't care about, we should expect to approach 100% code coverage in order to have a good chance of producing well-tested, maintainable code that gives us the flexibility and confidence to refactor ruthlessly.
Code Coverage sometimes gets a bad reputation because it can be easy to game. That is, it is not a good metric of success for a development team. However, it is a magnificenttool to help you identify places where tests are missing. It won't tell you where your tests are not doing what they need to do, but it will tell you when you have a piece of code that is not exercised by any tests. If you are a TDD (Test-Driven-Development) practitioner, as I am, Code Coverage will tell you when you’ve gotten ahead of yourself and written code before writing a test for it. This is especially valuable for those who are just learning TDD, but never loses its value no matter how experienced you are at TDD.
Continue the Conversation
How do you feel about this logic? Did I miss something critical in this analysis? Have you found something different in your experience? Let's keep this conversation going on Twitter. Tweet me @bsstahl with your comments, or post on your blog and tweet me the link.