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

Marc is almost certainly correct. Here's a way to verify

1. Open Reflector.exe
2. Add all of Non-System assemblies
3. F3 and search for ExtensionAttribute

If it pops up anywhere besides System.Core then you know where it's coming from.