C# – strange warning about ExtensionAttribute

ccompiler-warnings

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 Files\Reference Assemblies\Microsoft\Framework\v3.5\System.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

Best Answer

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.

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