Could not load type 'System.Runtime.CompilerServices.ExtensionAttribute' from assembly 'mscorlib, Version=4.0.0.0 Error

This error comes about when you compile an application against .Net 4.5 but then run it on a machine that only has 4.0 installed.

In .Net 4.5 the ExtensionAttribute class was moved from System.Core to mscorlib. There is a type forwarder in System.Core that points to the new location in mscorlib. If you compile an app against 4.5 it will expect to find ExtensionAttribute in mscorlib. If you then run it against 4.0 you will get this exception because in actually lives in System.Core in 4.0

To fix this either deploy 4.5 to the server or make sure that you build against 4.0