Can't get xUnit tests to run with .NET Core

Changing the test project from Class Library (.NET Standard) to Class Library (.NET Core) fixed the problem. I still don't get why this is supposed to make a difference.

Unit tests are applications that we run. To build such an application, we must specify a runtime and application model. When we target .NET Standard, the runtime and application model are ambiguous; MSBuild does not know whether to build against .NET Framework, .NET Core, Mono/Xamarin, or another .NET Standard compliant platform. Targeting .NET Core provides the required input to MSBuild, which now knows how to resolve all the referenced assemblies/projects and choose the appropriate framework version.

But that was never the case. In the past, you would always use a class library for unit tests; it was never considered an application that you could run, but rather a collection of classes and methods that you would feed into a test runner.

In the past we did not have .NET Standard, which is an ambiguous target. When MSBuild sees .NET Standard it needs more information. "Okay, which .NET Standard compliant runtime do you want to use to produce the runnable output?" If for instance we had targeted netstandard1.2, then MSBuild would not know whether to build against .NET Core 1.0, .NET Framework 4.5.1, Windows 8.1, or the several other netstandard1.2 compliant platforms.

Starting test execution, please wait... Could not find testhost.dll for source '[...].Tests.dll'. Make sure test project has a nuget reference of package "microsoft.testplatform.testhost".

If we do not specify netcoreapp, then MSBuild assumes that we are using the full framework. In that case, it expects the target assemblies, including testhost.dll, to be in the bin. If they are not (and they will not be if we built against .NET Standard), then we will receive the above error.

My issue was the same, with message in Output > Test console:

Microsoft.VisualStudio.TestPlatform.ObjectModel.TestPlatformException:
Unable to find [test-dir]\bin\Debug\netcoreapp3.1\testhost.dll.
Please publish your test project and retry.

I have xUnit test based on packages:

• xunit (2.4.1)
• xunit.runner.visualstudio (2.4.2)

What helps me:

• remove package: Microsoft.NET.Test.Sdk
• install package: Microsoft.TestPlatform.TestHost

Now works and even faster !

All xUnit based Test Project packages in VS.NET:

In NuGet Package Manager: