Overthinking CSV With Cesil: Testing And Code Coverage in 2020

I’ve always been a proponent of extensive test suites, especially for libraries. Accordingly, Cesil has a lot of tests – 665 at time of writing. I also decided to set some code coverage goals: covering 90+% of lines and 80+% of branches. None of this is new to modern .NET, though there have been some small changes over the years.

The big one compared to some of my older projects is that, since .NET Standard 2.1 in 2018, .NET has a standard way to run tests – dotnet test. I used to always write up a small test runner project so I could easily run tests without other tooling (like Visual Studio’s test runner), but that is no longer necessary with this new command.

I’ve also moved away from the testing framework that shipped with Visual Studio (Microsoft.VisualStudio.TestTools.UnitTesting) and am now using xUnit. There’s no loss in functionality (Visual Studio and dotnet test understand xUnit out of the box), and I find that xUnit is just generally more popular. The final straw was Stack Overflow (my day job) adopting xUnit for their internal tests. If this sounds like a mediocre endorsement, it sort of is – I don’t get super excited about unit testing frameworks provided they check the appropriate boxes. xUnit gives you asserts, parameterization, and decent logging – it’ll do.

While you don’t get code coverage out of the box, it has also gotten a lot easier. Coverlet is a nuget package that, once installed, lets you just add “/p:CollectCoverage=true” to “dotnet test” and get line and branch coverage statistics from your test suite. Results can be exported in a variety of formats, but for Cesil I’ve stuck with ReportGenerator.

This is a notable improvement from OpenCover, what I had been using for code coverage (even early versions of Cesil still used OpenCover). Coverlet is considerably less fiddly and (at least in theory) supports non-Windows platforms.

This modern tooling also plays nicely with GitHub Actions, which let me automate reporting code coverage statistics. Now whenever a change hits the appropriate branches, a full run of tests is kicked off and results are automatically extracted into little badges that appear in Cesil’s README.

A slightly clever trick is using a separate branch (the shields branch, in Cesil’s case) to hold code coverage results – alleviating the need to store any data outside of the repository. The whole dance can be seen in the CodeCoverageBadges.yml file that runs on check in.


When aiming for high rates of code coverage you have to figure out patterns for making sure rarely taking code paths are visited. In Cesil these rare paths are those caused by restricted allocations, async optimizations, and cancellation. I dealt with these by introducing various interfaces only implemented during DEBUG builds, and then making extensive use of helpers in tests that use those interfaces to run the test multiple times for each relevant code path. A concrete example is a good illustration of what I’m talking about.

  1. Most async methods in Cesil have optimizations for when a (Value)Task returned by a relied upon method has already completed.
  2. If at any point in this fast path a task has not yet completed, Cesil will switch to an implementation that actually awaits the task.
  3. This means that there is a (hopefully rarely taken) branch in the fast path after every (Value)Task returning method is called.
  4. Each class with methods like this implements ITestableAsyncProvider, and calls AsyncTestHelper.IsCompletedSuccessfully() instead of (Value)Task.IsCompletedSuccessfully.
  5. In Cesil.Tests, the various RunAsyncXXXVariants() methods repeatedly re-run the actual tests (wrapped as delegates) using ITestableAsyncProvider to signal that IsCompletedSuccesfully() should return false after an explicit number of calls.
  6. By increasing the switch-to-async change over point we can ensure that the test explores all branches introduced by this sort of async optimization.


I don’t have a way to signal that certain statements are unreachable. That may sound like a weird problem to have, but Cesil employs the Throw Helper pattern so it’s commonly found in any error branches. I alleviate it some by having the methods return a value, so I can write return Throw.BlahBlah(…) instead but in void returning methods you still see uncovered, yet unreachable, lines like this one:

It’d be useful if I could signal to a code coverage tool that certain methods never return, and thereby remove these lines from Cesil’s metrics. .NET already has attributes for that purpose, but Coverlet does not support them – I’ve opened an issue to see if this can be improved.

As you can see, Cesil has pretty extensive tests but there are still some real deficiencies. Naturally, that means the Open Question is: Are there any changes I should make to improve Cesil’s testing?

As before, I’ve opened an issue to gather long form responses.  Remember that, as part of the sustainable open source experiment I detailed in the first post of this series, any commentary from a Tier 2 GitHub Sponsor will be addressed in a future comment or post. Feedback from non-sponsors will receive equal consideration, but may not be directly addressed.

The next post in this series will cover Cesil’s documentation.