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

Posted written by Paul Seal on March 08, 2019 .NET Framework

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