title: Type or namespace name does not exist
body: <p>I have a WCF Data Service project built with Visual Studio 2010, which was working fine. All of a sudden, it didn't compile anymore. It was giving me messages like:</p>
  <blockquote>
  <p>Error 7   The type or namespace name 'Services' does not exist in the namespace 'System.Data' (are you missing an assembly reference?)    C:\U...s\Visual Studio 2010\Projects...\DataService.cs ...</p>
  <p>Error  8   The type or namespace name 'Linq' does not exist in the namespace 'System' (are you missing an assembly reference?) DependencyResolver.cs   3   14</p>
  <p>Error  10  The type or namespace name 'Web' does not exist in the namespace 'System.ServiceModel' (are you missing an assembly reference?)</p>
  <p>Error  12  The type or namespace name 'DataService' could not be found (are you missing a using directive or an assembly reference?)</p>
  </blockquote>
  <p>How can I fix it?</p>