unable to access the server.mappath() methode what is the wrong here

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
namespace AjaxControlsDemonstration
{
    public class serviceForAutoComplete
    {
        public List getCountires(string x)
        {
            List countries = new List();
           
            DataSet ds = new DataSet();
            ds.ReadXml();
            return countries;
        
        }
    }
}