error in wcf

Hi this is my wcf application
[ServiceContract]
    public interface IService1
    {

        [OperationContract]
        IEnumerable<Department> depts();
        // TODO: Add your service operations here
    }

//implementation
public class Service1 : IService1
    {
public IEnumerable<Department> depts()
        {
            XDocument xdoc = XDocument.Load(HttpContext.Current.Server.MapPath("Dept.xml"));
            IEnumerable<Department> depts = from p in xdoc.Descendants("Department")
              & ...