C# 2008 clear out multiple assemblies in the global alias

Question

I am getting an error in a C# 2008 application since I am trying to add  the NLog open  source tool to 2  different solution files.

I am completing the following task:

1. I add a reference  to this dll in the references section of each proejct file.

2. I  then place a 'using NLog' statment in each solution file.

No matter what I do, I can not remove 'System.Runtime.CompilerServices.ExtensionAttribute' is defined in multiple  assemblies in the global alias;

Can you show me  code and/or tell me how to fix this problem?I am getting the following warning and error message for a C# 2008 application I am working on:

Warning 17 The predefined type 'System.Runtime.CompilerServices.ExtensionAttribute' is defined in multiple assemblies in the global alias;
using definition from 'c:\Program Files (x86)\NLog\.NET Framework 2.0\NLog.dll' sample

Error 21 The type or namespace name 'Nlog' could not be found (are you missing a using directive or an assembly reference?) C:\Sample.cs 15 7 sample