Unit testing private code

Question

Hello all

I am currently involved in developing with C# - Here is some background: We implement MVP with our client application and we have a cyclomatic rule which states that no method should have a cyclomatic complexity greater than 5. This leads to a lot of small private methods which are generally responsible for one thing.

My question is about unit testing a class:

Testing the private implementation through the public methods is all fine... I don't have a problem implementing this.

But... what about the following cases:

Example 1. Handle the result of an async data retrival request (The callback method shouldn't be public purely for testing)

Example 2. An event handler which does an operation (such as update a View label's text - silly example I know...)

Example 3. You are using a third party framework which allows you to extend by overriding protected virtual methods (the path from the public methods to these virtual methods are generally treated as black box programming and will have all sorts of dependancies that the framework provides that you don't want to know about)

The examples above don't appear to me to be the result of poor design. They also do not appear be be candidates for moving to a seperate class for testing in isolation as such methods will lose their context.

Doesn anyone have any thoughts about this?

Cheers, Jason

EDIT: I don't think I was clear enough in my original question - I can test private methods using accessors and mock out calls/ methods using TypeMock. That isn't the problem. The problem is testing things which don't need to be public, or can't be public.

I don't want to make code public for the sake of testing as it can introduce security loopholes (only publishing an interface to hide this is not an option because anyone can just cast the object back to its original type and get access to stuff I wouldn't want them to)

Code that gets refactored out to another class for testing is fine - but can lose context. I've always thought it bad practice to have 'helper' classes which can contain a pot of code with no specific context - (thinking SRP here). I really don't think this works for event handlers either.

I am happy to be proven wrong - I just am unsure how to test this functionality! I have always been of the mind that if it can break or be changed - test it.

Cheers, Jason

Answer 1

In all of my unit testing, I've never bothered testing private functions. I typically just tested public functions. This goes along with the Black Box Testing methodology.

You are correct that you really can't test the private functions unless you expose the private class.

If your "seperate class for testing" is in the same assembly, you can choose to use internal instead of private. This exposes the internal methods to your code, but they methods will not be accessible to code not in your assembly.

EDIT: searching SO for this topic I came across this question. The most voted answer is similar to my response.

Answer 2

I will admit that when recently writing units tests for C# I discovered that many of the tricks I knew for Java did not really apply (in my case it was testing internal classes).

For example 1, if you can fake/mock the data retrieval handler you can get access to the callback through the fake. (Most other languages I know that use callbacks also tend not to make them private).

For example 2 I would look into firing the event to test the handler.

Example 3 is an example of the Template Pattern which does exist in other languages. I have seen two ways to do this:

1. Test the entire class anyway (or at least relevant pieces of it). This particularly works in cases where the abstract base class comes with its own tests, or the overall class is not too complex. In Java I might do this if I were writing an extension of AbstractList, for example. This may also be the case if the template pattern was generated by refactoring.

2. Extend the class again with extra hooks that allow calling the protected methods directly.

Answer 3

As Chris has stated, it is standard practice to only unit test public methods. This is because, as a consumer of that object, you are only concerned about what is publically available to you. And, in theory, proper unit tests with edge cases will fully exercise all private method dependencies they have.

That being said, I find there are a few times where writing unit tests directly against private methods can be extremely useful, and most succinct in explaining, through your unit tests, some of the more complex scenarios or edge cases that might be encountered.

If that is the case, you can still invoke private methods using reflection.

MyClass obj = new MyClass();
MethodInfo methodInfo = obj.GetType().GetMethod("MethodName", BindingFlags.Instance | BindingFlags.NonPublic);
object result = methodInfo.Invoke(obj, new object[] { "asdf", 1, 2 });
// assert your expected result against the one above

Answer 4

we have a cyclomatic rule which states that no method should have a cyclomatic complexity greater than 5

I like that rule.

The point is that the private methods are implementation details. They are subject to change/refactoring. You want to test the public interface.

If you have private methods with complex logic, consider refactoring them out into a separate class. That can also help keep cyclomatic complexity down. Another option is to make the method internal and use InternalsVisibleTo (mentioned in one of the links in Chris's answer).

The catches tend to come in when you have external dependencies referenced in private methods. In most cases you can use techniques such as Dependency Injection to decouple your classes. For your example with the third-party framework, that might be difficult. I'd try first to refactor the design to separate the third-party dependencies. If that's not possible, consider using Typemock Isolator. I haven't used it, but its key feature is being able to "mock" out private, static, etc. methods.

Classes are black boxes. Test them that way.

EDIT: I'll try to respond to Jason's comment on my answer and the edit to the original question. First, I think SRP pushes towards more classes, not away from them. Yes, Swiss Army helper classes are best avoided. But what about a class designed to handle async operations? Or a data retrieval class? Are these part of the responsibility of the original class, or can they be separated?

For example, say you move this logic to another class (which could be internal). That class implements an Asynchronous Design Pattern that permits the caller to choose if the method is called synchronously or asynchronously. Unit tests or integration tests are written against the synchronous method. The asynchronous calls use a standard pattern and have low complexity; we don't test those (except in acceptance tests). If the async class is internal, use InternalsVisibleTo to test it.

Answer 5

There is really only two cases you need to consider:

1. the private code is called, directly or indirectly from public code and
2. the private code is not called from public code.

In the first case, the private code is automatically being tested by the tests which exercise the public code that calls the private code, so there is no need to test the private code. And in the second case, the private code cannot be called at all, therefore it should be deleted, not tested.

Ergo: there is no need to explicitly test the private code.

Note that when you do TDD it is impossible for untested private code to even exist. Because when you do TDD, the only way that private code can be appear, is by an Extract {Method|Class|...} Refactoring from public code. And Refactorings are, by definition, behavior-preserving and therefore test-coverage-preserving. And the only way that public code can appear is as the result of a failing test. If public code can only appear as already tested code as the result of a failing test, and private code can only appear as the result of being extracted from public code via a behavior-preserving refactoring, it follows that untested private code can never appear.

Answer 6

A few points from a TDD guy who has been banging around in C#:

1) If you program to interfaces then any method of a class that is not in the interface is effectively private. You might find this a better way to promote testability and a better way to use interfaces as well. Test these as public members.

2) Those small helper methods may more properly belong to some other class. Look for feature envy. What may not be reasonable as a private member of the original class (in which you found it) may be a reasonable public method of the class it envies. Test these in the new class as public members.

3) If you examine a number of small private methods, you might find that they have cohesion. They may represent a smaller class of interest separate from the original class. If so, that class can have all public methods, but be either held as a private member of the original class or perhaps created and destroyed in functions. Test these in the new class as public members.

4) You can derive a "Testable" class from the original, in which it is a trivial task to create a new public method that does nothing but call the old, private method. The testable class is part of the test framework, and not part of the production code, so it is cool for it to have special access. Test it in the test framework as if it were public.

All of these make it pretty trivial to have tests on the methods that are currently private helper methods, without messing up the way intellisense works.

Answer 7

Would accessor files work? http://msdn.microsoft.com/en-us/library/bb514191.aspx I've never directly worked with them, but I know a coworker used them to test private methods on some Windows Forms.

Answer 8

Don't test private code, or you'll be sorry later when it's time to refactor. Then, you'll do like Joel and blog about how TDD is too much work because you constantly have to refactor your tests with your code.

There are techniques (mocks, stub) to do proper black box testing. Look them up.

Answer 9

Several people have responded that private methods shouldn't be tested directly, or they should be moved to another class. While I think this is good, sometimes its just not worth it. While I agree with this in principle, I've found that this is one of those rules that cna be broken to save time without negative repercussions. If the function is small/simple the overhead of creating another class and test class is overkill. I will make these private methods public, but then not add them to the interface. This way consumers of the class (who should be getting the interface only through my IoC library) won't accidentally use them, but they're available for testing.

Now in the case of callbacks, this is a great example where making a private property public can make tests a lot easier to write and maintain. For instance, if class A passes a callback to class B, I'll make that callback a public property of class A. One test for class A use a stub implementation for B that records the callback passed in. The test then verify the the callback is passed in to B under appropriate conditions. A separate test for class A can then call the callback directly, verifying it has the appropriate side effects.

I think this approach works great for verifying async behaviors, I've been doing it in some javascript tests and some lua tests. The benefit is I have two small simple tests (one that verifies the callback is setup, one that verifies it behaves as expected). If you try to keep the callback private then the test verifying the callback behavior has a lot more setup to do, and that setup will overlap with behavior that should be in other tests. Bad coupling.

I know, its not pretty, but I think it works well.

Answer 10

This is a question that comes up pretty early when introducing testing. The best technique to solving this problem is to black-box test (as mentioned above) and follow the single responsibility principle. If each of your classes only have only one reason to change, they should be pretty easy to test their behavior without getting at their private methods.

SRP - wikipedia / pdf

This also leads to more robust and adaptable code as the single responsibility principle is really just saying that your class should have high cohesion.

Answer 11

There are some great answers here, and I basically agree with the repeated advice to sprout new classes. For your Example 3, however, there's a sneaky, simple technique:

Example 3. You are using a third party framework which allows you to extend by overriding protected virtual methods (the path from the public methods to these virtual methods are generally treated as black box programming and will have all sorts of dependencies that the framework provides that you don't want to know about)

Let's say MyClass extends FrameworkClass. Have MyTestableClass extend MyClass, and then provide public methods in MyTestableClass that expose the protected methods of MyClass that you need. Not a great practice - it's kind of an enabler for bad design - but useful on occasion, and very simple.