System.Xml.Linq.XDocument'在未引用的程序集中定义

I have a solution which contains a few projects, one being a web sevrice, the others being class libraries. I want to use Xdocuments in both the web service and one of the class libraries, so i have added a reference to System.xml.Linq in solution explorer, and included it within the relevant .cs files by using System.Xml.Linq. This is also reference in the web.config as follows:I have a solution which contains a few projects