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

Another easy way to verify: In your code, temporarily use the class somewhere. Example:

System.Runtime.CompilerServices.ExtensionAttribute x = null;

When building, this will generate error:

The type 'System.Runtime.CompilerServices.ExtensionAttribute' exists in both 'c:\Program Files\Reference Assemblies\Microsoft\Framework\v3.5\System.Core.dll' and .....

And show you immediately the 2 sources causing the conflict.