How to solve the error Could not load file or assembly System.Security.Principal.Windows

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:

  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.

Related posts


This is why you should never judge a book by its cover
How to create a Carousel in Umbraco using Nested Content and Bootstrap
Umbraco 9 Useful Snippets
How to render the actual document type icons in the tree in Umbraco

Proudly sponsored by

Moriyama
  • Moriyama build, support and deploy Umbraco, Azure and ASP.NET websites and applications.
Find out more
AppVeyor
  • CI/CD service for Windows, Linux and macOS
  • Build, test, deploy your apps faster, on any platform.
Find out more
elmah.io
  • elmah.io is the easy error logging and uptime monitoring service for .NET.
  • Take back control of your errors with support for all .NET web and logging frameworks.
Find out more
uSync Complete
  • uSync.Complete gives you all the uSync packages, allowing you to completely control how your Umbraco settings, content and media is stored, transferred and managed across all your Umbraco Installations.
Find out more
uSkinned
  • More than a theme for Umbraco CMS, take full control of your content and design with a feature-rich, award-nominated & content editor focused website platform.
Find out more
UmbHost
  • Affordable, Geo-Redundant, Umbraco hosting which gives back to the community by sponsoring an Umbraco Open Source Developer with each hosting package sold.
Find out more