Try stripping away everything but the Assembly Name from the project reference in the project file. (E.g. System.IO.FileSystem, Version=4.0.2.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL => System.IO.FileSystem.) It looks like the wrong version of IO.FileSystem is present (It's trying to load 4.0.1 with a 4.0.2 reference with the 4.3.0 package). It's possible that allowing multiple versions would cause more trouble, but I don't think you're doing anything complicated enough with it to matter.