Visual Studio 2015 or 2017 does not discover unit tests

It could be that your code is compiled with x64 therefore have to enable the Default Processor Architecture as X64.

Test > Test Settings > Default Processor Architecture > X64

EDIT 2016-10-19 (PowerShell script)

This issue still returns every now and then. I wrote a small PowerShell snippet to automate clearing the relevant cache/temp folder/files for me. I'm sharing it here for future readers:

@(
"$env:TEMP"
"$env:LOCALAPPDATA\Microsoft\UnitTest"
"$env:LOCALAPPDATA\Microsoft\VisualStudio\14.0\1033\SpecificFolderCache.xml"
"$env:LOCALAPPDATA\Microsoft\VisualStudio\14.0\1033\ProjectTemplateMRU.xml"
"$env:LOCALAPPDATA\Microsoft\VisualStudio\14.0\ComponentModelCache"
"$env:LOCALAPPDATA\Microsoft\VisualStudio\14.0\Designer\ShadowCache"
"$env:LOCALAPPDATA\Microsoft\VisualStudio\14.0\ImageLibrary\cache"
"$env:LOCALAPPDATA\Microsoft\VisualStudio Services\6.0\Cache"
"$env:LOCALAPPDATA\Microsoft\WebsiteCache"
"$env:LOCALAPPDATA\NuGet\Cache"
) |% { Remove-Item -Path $_ -Recurse -Force }

Make sure to close Visual Studio beforehand and it's probably a good idea to reboot afterwards.

Deleting the TEMP folder may not be necessary and may in some cases even be undesirable, so I would recommend trying without clearing the TEMP folder first. Just omit the "$env:TEMP".

Original answer 2015-04-12

The problem was "solved" after a thorough cleaning of Visual Studio-related temp/cache folders.

Since I did not have the time to go through everything one-by-one and then test in-between, I unfortunately don't know which one actually caused the problem.

These are the exact steps I've taken:

1. Closed Visual Studio
2. Used CCleaner to clear system and browser temp files/folders

3. Manually cleared/deleted the following files/folders:

   • %USERPROFILE%\AppData\Local\assembly
   • %USERPROFILE%\AppData\Local\Microsoft\UnitTest
   • %USERPROFILE%\AppData\Local\Microsoft\VisualStudio\14.0\1033\SpecificFolderCache.xml
   • %USERPROFILE%\AppData\Local\Microsoft\VisualStudio\14.0\1033\ProjectTemplateMRU.xml
   • %USERPROFILE%\AppData\Local\Microsoft\VisualStudio\14.0\ComponentModelCache
   • %USERPROFILE%\AppData\Local\Microsoft\VisualStudio\14.0\Designer\ShadowCache
   • %USERPROFILE%\AppData\Local\Microsoft\VisualStudio\14.0\ImageLibrary\cache
   • %USERPROFILE%\AppData\Local\Microsoft\VisualStudio Services\6.0\Cache
   • %USERPROFILE%\AppData\Local\Microsoft\WebsiteCache
   • %USERPROFILE%\AppData\Local\NuGet\Cache
   • %USERPROFILE%\AppData\Local\Temp

• Check out, if NUnit Test Adapter 2/3 is installed in VisualStudio.
  (Tools>Extensions and Updates )

• Make sure that correct processor architecture is chosen:
  (Test>Test Settings>Default Processor Architecture)

To my surprise, clearing temp files located in the %TEMP% directory resolved the issue for me.

Note: This path is generally at C:\Users\(yourusername)\AppData\Local\Temp

As @Warren-P included, you can navigate to the temp folder by putting in %temp% in Start Menu, or launch "File Explorer" and enter %temp% in the address bar.