Unable to find testhost.dll. Please publish your test project and retry

Installing Microsoft.NET.Test.Sdk package from nuget package manager solved my issue.

In my case, the problem was that I was targeting .NET Core 2.0 and switching to .NET Core 2.1 solved the problem. However I was using Microsoft.NET.Test.SDK v16.4.0 instead of 15.9.0.

I had created a class library and tried to use the XUnit NuGet package in it.

What I should have done was created an XUnit project using this command: dotnet new xunit -n TestProject

I found this helpful page.

In my case the problem was that I have an extension project for xunit. There is also a test project to test the extensions. When I ran dotnet test on my solution, my extension project was also picked up as a unit test project (it took me some time to realize this). The reason for this is that it references some xunit packages. One of these xunit packages automatically sets the <IsTestProject>true</IsTestProject> property in you csprj file. This is actually a good thing since 99.99% of the projects that reference xunit are actually unit tests. I could finally solve this by explicitly setting

     <PropertyGroup>
...
        <IsTestProject>false</IsTestProject>
...
      </PropertyGroup>

Manually in my csproj file. Then the problem went away.