title: The primary reference "Microsoft.CSharp", which is a framework assembly, could not be resolved in the currently targeted framework
body: <p>I started a new winforms project, fairly simple, has a few labels, text boxes, and a button. All code is running fine. I forgot to change the framework version from 4 (using VS 2010) to 3.5 before I created the app and when I changed it to 3.5 or 3.5 client profile I get the following warning:</p>
  
  <blockquote>
    <p>The primary reference "Microsoft.CSharp", which is a framework assembly, could not be  resolved in the currently targeted framework. ".NETFramework,Version=v3.5". To resolve this problem, either remove the reference "Microsoft.CSharp" or retarget your application to a framework version which contains "Microsoft.CSharp".</p>
  </blockquote>
  
  <p>It's not a huge amount of trouble to recreate this with a 3.5 setting from the start and I'm fairly sure I'm not using anything from the 4 framework that I need.</p>