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:

  1. Search for Microsoft.Net.Compilers
  2. Select the projects which do not have the project installed yet
  3. Choose the same version that is already installed on the other project
  4. Click Install

Once you have installed it. You should be able to do a clean and then a rebuild without the error now.

Paul Seal

Umbraco MVP and .NET Web Developer from Derby (UK) who specialises in building Content Management System (CMS) websites using MVC with Umbraco as a framework. Paul is passionate about web development and programming as a whole. Apart from when he's with his wife and son, if he's not writing code, he's thinking about it or listening to a podcast about it.

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