title: Backwards Compatibility of .NET Framework 4
body: <p>We have an WPF Application build on .net framework 3.5.</p>
  
  <p>Some testers find if they uninstall .net framework 3.5, but install .net framework 4.0, our APP fails to launch itself.</p>
  
  <p>Dose this mean that .net framework 4.0 does not include all 3.5 libs, and users have to install .net 3.5 even though they have 4.0?</p>
  
  <p>I see here are some migration issues listed by Microsoft
  <a href="http://msdn.microsoft.com/en-us/library/ee941656.aspx#windows_presentation_foundation_wpf" rel="noreferrer">http://msdn.microsoft.com/en-us/library/ee941656.aspx#windows_presentation_foundation_wpf</a></p>
  
  <p>Are they all breaking changes so that the backward compatibility is ruined?</p>
  
  <p>Thanks</p>