title: The file could not be added to the project. This file is within the project directory tree error
body: <p>Whenever I try to add a file to my Visual Studio project, I get the following warning:</p>
  
  <blockquote>
    <p>The file [filename] could not be added to the project. This file is within the project directory tree</p>
  </blockquote>
  
  <p>How do I fix this error?</p>