How should I unit test multithreaded code?
Look, there's no easy way to do this. I'm working on a project that is inherently multithreaded. Events come in from the operating system and I have to process them concurrently.
The simplest way to deal with testing complex, multithreaded application code is this: If it's too complex to test, you're doing it wrong. If you have a single instance that has multiple threads acting upon it, and you can't test situations where these threads step all over each other, then your design needs to be redone. It's both as simple and as complex as this.
There are many ways to program for multithreading that avoids threads running through instances at the same time. The simplest is to make all your objects immutable. Of course, that's not usually possible. So you have to identify those places in your design where threads interact with the same instance and reduce the number of those places. By doing this, you isolate a few classes where multithreading actually occurs, reducing the overall complexity of testing your system.
But you have to realize that even by doing this, you still can't test every situation where two threads step on each other. To do that, you'd have to run two threads concurrently in the same test, then control exactly what lines they are executing at any given moment. The best you can do is simulate this situation. But this might require you to code specifically for testing, and that's at best a half step towards a true solution.
Probably the best way to test code for threading issues is through static analysis of the code. If your threaded code doesn't follow a finite set of thread safe patterns, then you might have a problem. I believe Code Analysis in VS does contain some knowledge of threading, but probably not much.
Look, as things stand currently (and probably will stand for a good time to come), the best way to test multithreaded apps is to reduce the complexity of threaded code as much as possible. Minimize areas where threads interact, test as best as possible, and use code analysis to identify danger areas.
It's been a while when this question was posted, but it's still not answered ...
kleolb02's answer is a good one. I'll try going into more details.
There is a way, which I practice for C# code. For unit tests you should be able to program reproducible tests, which is the biggest challenge in multithreaded code. So my answer aims toward forcing asynchronous code into a test harness, which works synchronously.
It's an idea from Gerard Meszardos's book "xUnit Test Patterns" and is called "Humble Object" (p. 695): You have to separate core logic code and anything which smells like asynchronous code from each other. This would result to a class for the core logic, which works synchronously.
This puts you into the position to test the core logic code in a synchronous way. You have absolute control over the timing of the calls you are doing on the core logic and thus can make reproducible tests. And this is your gain from separating core logic and asynchronous logic.
This core logic needs be wrapped around by another class, which is responsible for receiving calls to the core logic asynchronously and delegates these calls to the core logic. Production code will only access the core logic via that class. Because this class should only delegate calls, it's a very "dumb" class without much logic. So you can keep your unit tests for this asychronous working class at a minimum.
Anything above that (testing interaction between classes) are component tests. Also in this case, you should be able to have absolute control over timing, if you stick to the "Humble Object" pattern.
Tough one indeed! In my (C++) unit tests, I've broken this down into several categories along the lines of the concurrency pattern used:
Unit tests for classes that operate in a single thread and aren't thread aware -- easy, test as usual.
Unit tests for Monitor objects (those that execute synchronized methods in the callers' thread of control) that expose a synchronized public API -- instantiate multiple mock threads that exercise the API. Construct scenarios that exercise internal conditions of the passive object. Include one longer running test that basically beats the heck out of it from multiple threads for a long period of time. This is unscientific I know but it does build confidence.
Unit tests for Active objects (those that encapsulate their own thread or threads of control) -- similar to #2 above with variations depending on the class design. Public API may be blocking or non-blocking, callers may obtain futures, data may arrive at queues or need to be dequeued. There are many combinations possible here; white box away. Still requires multiple mock threads to make calls to the object under test.
As an aside:
In internal developer training that I do, I teach the Pillars of Concurrency and these two patterns as the primary framework for thinking about and decomposing concurrency problems. There's obviously more advanced concepts out there but I've found that this set of basics helps keep engineers out of the soup. It also leads to code that is more unit testable, as described above.