Expecting json/xml data in webapi

try
      {
 string oRequest = _xml.UpdateInvereqxml(UserName, Password,OTAhotelid, Invy);
     string uri = @"service/update";
         System.Net.WebRequest req = System.Net.WebRequest.Create(uri);
         req.Method = "POST";
         req.ContentType = "text/xml";
         System.IO.StreamWriter writer = new System.IO.StreamWriter(req.GetRequestStream());
         writer.WriteLine(oRequest);
         writer.Close();
         System.Net.WebResponse rsp = req.GetResponse();
         Stream istrm = rsp.GetResponseStream();
         string StreamReader = new StreamReader(istrm).ReadToEnd();
     }