c# - strange warning about ExtensionAttribute

Question

I'm getting a strange warning:

The predefined type 'System.Runtime.CompilerServices.ExtensionAttribute' is defined in multiple assemblies in the global alias; using definition from 'c:Program FilesReference AssembliesMicrosoftFrameworkv3.5System.Core.dll'

There is no line number given, so it's hard to figure out what it's on about.

The compiler error code is CS1685

Answer 1

Are you using someone's dll (or your own) which had implemented this attribute (with exactly the same name) itself as a means of using some c# 3.0 features on pre .Net 3.5 runtimes? (A common trick)

This is the probable cause. Since it is using the correct one (the MS one in the GAC) this is not a problem though you should hunt down the other and remove it.