.NET Standard gotcha: unit tests don't need to be Standard

You created a library in .NET Core and you want it to be usable for .NET Framework or Xamarin as well, so you want to turn it into a .NET Standard library. You learn that it is simple: a matter of changing the TargetFramework in the .csproj file, so you do this for all projects in the solution.

But .NET Standard is only designed for libraries, so it makes no sense to change the TargetFramework for other types of projects, including some that are in fact libraries, but are not used as such, like unit test projects.

For example if you attempt to run XUnit tests in Visual Studio you will see that the tests are discovered by Test Explorer, but when you try to run them it says "No test is available in [your project]. Make sure that test discoverer & executors are registered and platform & framework version settings are appropriate and try again.". While this is an issue with XUnit, more likely, it is also a non-issue, since your test project should use the .NET Standard libraries, not be one itself.