If your web service needs information from some external files, a logical place for these files is the App_Data folder.

To get the fysical path of the file in a WCF service you can use this code:

RolemappingPath = HostingEnvironment.MapPath(@”/App_Data/RoleMapping.xml”);

The class HostingEnvironment is in the namespace System.Web.Hosting, and you will also need to add a reference to System.Web.dll.

Reference: http://msdn.microsoft.com/en-us/library/system.web.hosting.hostingenvironment.mappath.aspx