08 Mar 2019
I was getting this error when I had a web project and a class library project in the same solution.
System.IO.FileNotFoundException Could not load file or assembly System.Security.Principal.Windows
I wasn't able to build the class library project.
This was because the NuGet package Microsoft.Net.Compilers was not installed on both projects.
It was installed on the web project but not on the class library project.
So to fix this, go to Tools > NuGet Package Manager > Manage NuGet Packages for Solution
Then as the image shows, in the Installed tab:
Once you have installed it. You should be able to do a clean and then a rebuild without the error now.