namespace can't be found by omnisharp even though unity builds

Question

I am using vscode to edit unity scripts, I've added Mirror for needs, which extends its own namespace, but the problem is that omnisharp can't find it's namespace. All Mirror's files are under Assets/Thirdparty/Mirror

Everything works in terms of compiling and the result, but I have no idea how omnisharp works and how to configure it.

Below you can see waving lines at everything that is extended by the namespace.

Answer 1

Sometimes I have that too, especially when I just updated VS Code or added a package in Unity. And sometimes this helps:

Press the Regenerate project files button in Project Preferences.

Answer 2

For me installing .NET Framework 4.7.1 Developer Pack fixed the problem.