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.

Want to thank me?

If I've helped you out and you want to thank me, why not buy me a coffee?

About the author

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

How to set the default page base type to UmbracoViewPage in Umbraco

In this post I share with you what Ronald Barendse taught us about setting the pageBaseType in Umbra…

Read Post

How to fix corrupt accented characters in Excel for a C# export as CSV

This post will help you fix the issue where you csv export has corrupt characters when opening it in…

Read Post

How to create a dynamic image from multiple images using ImageProcessor

This post shows you how I created a handler to create dynamic header images for my umbraco website u…

Read Post

How to send Slack messages programmatically using C#

In this post I show you how you can send Slack messages programmatically using C#

Read Post