Unit Testing Asynchronous Code

摘要

Unit testing is a Cornerstone of modern development. The benefits of unit testing for a project are pretty well understood: Unit testing decreases the number of bugs, reduces time to market and discourages overly coupled design. Those are all nice benefits, but there are further advantages more directly relevant to developers. When I write unit tests, I can have much greater confidence in the code. It's easier to add features or fix bugs in tested code, because the unit tests act as a safety net while the code is changing. Writing unit tests for asynchronous code brings a few unique challenges. Furthermore, the current state of async support in unit test and mocking frameworks varies and is still evolving. This article will consider MSTest, NUnit and xUnit, but the general principles apply to any unit testing framework. Most of the examples in this article will use MSTest syntax, but I'll point out any differences in behavior along the way. The code download contains examples for all three frameworks. Before diving into the specifics, I'll briefly review a conceptual model of how the async and await keywords work.