C# Compiler Warning 1685

Question

So, (seemingly) out of the blue, my project starts getting compiler warning 1685:

The predefined type \'System.Runtime.CompilerServices.ExtensionAtt

Answer 1

FYI: I had the same problem and was able to resolve it by using Resharper's "Optimize References" command, and then removing all unused references. Not completely sure why that worked, but it did.