title: "Warning CS1684: Reference to type X claims it is defined in .. but it could not be found"
body: <p>I'm getting this warning when building a test project we'll call PWTests.</p>
  
  <blockquote>
    <p>Warning CS1684: Reference to type 'System.Windows.Input.ICommand' claims it is defined in 'c:\Program Files (x86)\Reference Assemblies\Microsoft\Framework.NETFramework\v4.0\System.dll', but it could not be found</p>
  </blockquote>
  
  <p>This project is supposed to test a class library which we'll call PW. I cannot determine why it thinks System.Windows.Input.ICommand is defined in System.dll. PresentationCore, where ICommand is actually defined, is properly referenced in the test project, and the class library project. I've tried removing the reference to PresentationCore, cleaning, and re-adding it (as mentioned in other questions) which did not work.</p>
  
  <p>How can I determine what is causing it to think that ICommand is in System.dll?</p>