How to fix Could not load file or assembly 'nunit.engine, Version=3.7.0.0

This is happening quite frequently with the latest versions of Visual Studio. We are not sure why, but the cause is an incomplete copy of the test adapter in the Visual Studio cache. The fix is to delete the cached copy and restart Visual Studio.

The folder you want to delete is C:\Users\username\AppData\Local\Temp\VisualStudioTestExplorerExtensions\NUnit3TestAdapter.3.x.x depending on your username and the version.

See https://github.com/nunit/nunit3-vs-adapter/issues/261 and http://hermit.no/how-to-resolve-cases-of-visual-studio-no-tests-appearing/

In my case it was

An exception occurred while invoking executor 'executor://nunit3testexecutor/': Could not load file or assembly 'nunit.engine, Version=3.9.0.0

And the problem is: in my test project I have an import of NUnit3TestAdapter.3.13.0

  <Import Project="..\..\..\packages\NUnit3TestAdapter.3.13.0\build\net35\NUnit3TestAdapter.props" Condition="Exists('..\..\..\packages\NUnit3TestAdapter.3.13.0\build\net35\NUnit3TestAdapter.props')" />

And in another project I have NUnit.Engine.3.10.0 referenced. but NUnit3TestAdapter.3.13.0 has Nunit.Engine.3.9.0 in the package.

\packages\NUnit3TestAdapter.3.13.0\build\net35

So, the solution was to downgrade Nunit.Engine in another project to 3.9 version.