title: Visual Studio doesn't release files used in WiX Extension
body: <p>I have a problem with building wixproj under Visual Studio.</p>
  
  <p>What I have done:</p>
  
  <ul>
  <li>added my preprocessor extension as separate project (wile reading <a href="https://stackoverflow.com/questions/5296179/wix-3-5-preprocessor-extension-undefined-preprocessor-function">this</a> post)</li>
  <li>added my extension method invocation in Product.wxs file</li>
  </ul>
  
  <p>First compilation works perfectly. I see changes made in Product.wxs. Things are going wrong while building second, third, fourth time and so on. </p>
  
  <p>This is what I get:</p>
  
  <blockquote>
    <p>"Unable to copy file
    "obj\x86\Release\WixStringFormattingExtension.dll" to
    "bin\Release\WixStringFormattingExtension.dll". The process cannot
    access the file 'bin\Release\WixStringFormattingExtension.dll' because
    it is being used by another process."</p>
  </blockquote>
  
  <p>Restarting Visual Studio helps ... but only for first compilation. It seems like something (light.exe candle.exe devenv.exe) is not releasing files.</p>