How to unit test a method that spawns threads?

Question

I am fairly new to TDD but have been using it for long enough to understand how to use mocks, stubs, dependency injection, inversion of control to solve 'similar' problems... but for some reason I feel very uneasy about using dependency injection and passing in an 'IThread' (or similar).

To give it some basic context - I am trying to add unit tests to a legacy application and I am unsure of how to unit test a class who's constructor spawns two threads.

Is the only option to use dependency injection?

If so what about the functionality that the threads bring? As it stands the threads both run while(true) loops and never exit the loop (unless the application is terminating). Inside the loops there are reasonable chunks of code and it's this code that I really want to have under test.

To make matters worse I don't want to pull all the functionality out of the loops and into public methods (I'm only testing public methods as my tests exist in another project) as it will really decrease the easy of use of the class elsewhere in the code.

Any suggestions?

Answer 1

Could you pull the functionality into internal methods and use InternalsVisibleTo instead? Even if you'd really want them to be private, this is a reasonable compromise solution.

If your threads would normally run forever, that does make it very hard to test... and it sounds like you really should be testing the "what the threads do" part separately, if they don't depend on being in separate threads.

One option which is occasionally useful is to have an IScheduler type of interface - ask that to execute an action wherever it sees fit; the production one would create a new thread, but your test one could run the action within the existing thread (or on a thread you had control over within your test code). I'm not sure that's appropriate for your situation here, where the thread would run forever, but you might want to think about it in other situatoins.