Visual Studio and Nuget based Test Runners/Adapters

Question

Recently I came to know about the nuget based adapter/runner (http://xunit.github.io/docs/running-tests-in-vs.html)

I was trying to understand how these runners/adapters are recognized by Visual Studio and then used by the TestWindow.

For example, if I install "xunit.runner.visualstudio" nuget package all the xUnit based tests in my project are discovered and displayed in the Test Window.

I am trying to understand how does VS hook the Test Discovery using the package installed in packages folder?

Where/how do VS and nuget based adapter get hooked?

I tried finding this information but my google fu failed me. :(

I checked answer for the question (Custom test adapter installed via NuGet isn't discovering tests) and it does say that VS copies the packages to the path %TEMP%\VisualStudioTestExplorerExtensions but that's pretty much it.

Answer 1

Since this involves a lot of how VS works internally, I don't think you'll get a totally definitive answer. However, to solve the issue you linked to I looked at a bunch of disassembled code, so I have a pretty good idea. Here's how it seems to work:

1. You build a project that references a package named Sample.TestAdapter
2. VS copies Sample.TestAdapter directory from packages to %TEMP%\VisualStudioTestExplorerExtensions
3. Something triggers test discovery - rebuild always does, sometimes incremental builds do too. vstest.console.exe is useful here for debugging.
4. VS launches vstest.discovery.exe which looks in VisualStudioTestExplorerExtensions for an assembly that implements ITestDiscoverer
5. If an ITestDiscoverer is found, VS calls it with a list of assemblies that may contain tests
6. Tests that are discovered are sent back to VS by your test adapter

So, as far as I can tell it's a pretty simple reflection based plug-in architecture. Hope that helps.