When modifying Script# source code, Resharper will incorrectly show errors for any reference to a class within the System namespace. Note that these Script# projects do not reference mscorlib.dll (and in fact this is checked in the project's Advanced Build Options property pane). Instead, all of the intrinsic System types are provided by a project reference to the Script# sscorlib.dll system assembly. I suspect that the underlying bug is that Resharper is always assuming that every C# project has a dependency on the mscorlib libraries as it's analyzing the source code, regardless of the project build settings. (BTW, if you're unfamiliar with Script#, you can download it here: http://projects.nikhilk.net/Projects/ScriptSharp.aspx)