title: How to include source files of one project in another project?
body: <p>I'm writing a class library in C#/.NET.</p>
  
  <p>I need to compile it for two different frameworks: </p>
  
  <ul>
  <li>4.0 (using for debug myself) </li>
  <li>3.5 (using on client).</li>
  </ul>
  
  <p>I want to have one set source files for the two projects, so I can make corrections in 1 copy of files and they are included in the other project automatically.</p>
  
  <p>Now, if I even use "add existing item", VS 2010 creates copies; and I need to copy the latest versions every time.</p>
  
  <p>I can't just change a target in project, because I'm using different versions of <code>.dll</code> references, and because ms vs has some quirks.</p>