namespace XXXSoft
{
    [ServiceContract]
    public interface IXXXSoft
    {
        [OperationContract]
        [ServiceKnownType(typeof(ConfigurationResponse))]
        [ServiceKnownType(typeof(Errors))]
        [WebInvoke(Method = "POST", ResponseFormat = WebMessageFormat.Xml, RequestFormat = WebMessageFormat.Xml, BodyStyle = WebMessageBodyStyle.Bare, UriTemplate = "Configuration")]
        object Configuration(ConfigurationRequest xml);
    }
}

namespace XXXSoft
{   
    public class XXXSoftService : IXXXSoft
    {
        public object Configuration(ConfigurationRequest xml)
        {
            tipoResult result = new tipoResult();
            Errors errors = new Errors();            
            ConfigurationResponse response = new ConfigurationResponse();            

            if (result.Result)
            {
                result = Users.Autorizzato(new tipoUser() {
                    Username = xml.Username,
                    Password = xml.Password
                });
                errors.Error = errors.Error.Union(result.Messaggi).ToList();

                if (result.Result)
                {
                    //Return Configuration
                }
            }

            if (errors.Error.Count > 0)
            {                
                return errors;
            }            

            return response;
        }        
    }
}

namespace XXXSoft
{
    [DataContract]    
    public class Errors
    {
        public Errors()
        {
            Error = new List<string>();
        }

        [DataMember]
        [XmlElementAttribute("Error", IsNullable = false)]
        public List<string> Error
        {
            get;

            set;
        }
    }
}

HttpWebRequest req = null;
            HttpWebResponse res = null;
            try
            {
                const string url = "http://localhost:54920/XXXSoft.svc/Configuration";
                req = (HttpWebRequest)WebRequest.Create(url);
                req.Method = "POST";
                req.ContentType = "application/xml; charset=utf-8";
                req.Timeout = 30000;
                req.Headers.Add("SOAPAction", url);

                var xmlDoc = new XmlDocument { XmlResolver = null };
                xmlDoc.Load(Server.MapPath("PostData.xml"));
                string sXml = xmlDoc.InnerXml;
                req.ContentLength = sXml.Length;
                var sw = new StreamWriter(req.GetRequestStream());
                sw.Write(sXml);
                sw.Close();

                res = (HttpWebResponse)req.GetResponse();
                Stream responseStream = res.GetResponseStream();
                var streamReader = new StreamReader(responseStream);

                //Read the response into an xml document
                var soapResonseXmlDocument = new XmlDocument();
                soapResonseXmlDocument.LoadXml(streamReader.ReadToEnd());

                //return only the xml representing the response details (inner request)
                string response = soapResonseXmlDocument.InnerXml;
                //Response.Write(soapResonseXMLDocument.InnerXml);
            }
            catch (Exception ex)
            {
                Response.Write(ex.Message);
            }

<Errors>
 <Error>Errore1</Error>
 <Error>Errore2</Error>
 <Error>Errore3</Error>
</Errors>

<Errors xmlns=\"http://schemas.datacontract.org/2004/07/EricSoft\" xmlns:i=\"http://www.w3.org/2001/XMLSchema-instance\">
  <Error xmlns:a=\"http://schemas.microsoft.com/2003/10/Serialization/Arrays\">
    <a:string>Error1</a:string>
  </Error>
  <Error xmlns:a=\"http://schemas.microsoft.com/2003/10/Serialization/Arrays\">
    <a:string>Error2</a:string>
  </Error>
  <Error xmlns:a=\"http://schemas.microsoft.com/2003/10/Serialization/Arrays\">
    <a:string>Error3</a:string>
  </Error>
</Errors>