WCF / ASMX :: Windows Communication Foundation Service Returning Xml / Json Data?

I Need by service contract to return the xml/json result depending on the request type. function which will convert my result set (i am using linq to sql) so that i do not need to create the xml format for the result set by iterating through the table row many times.What is the suitable way to do that.

I need a kind of short cut method which will convert the table data to xml result.Had i been using asp.net mvc i would have been able to generate the xml data by overriding the the ExecuteResult method in the ActionResult and giving Conetnt-Type = "text/xml" as OP.But since i am using Wcf i don't have the controller context(as controller context is the parameter that needs to be passed to Execute Result).

My present code for converting the table data to the xml format is below.

public XDocument UsersLists(string authToken)
{
bool IsAuthenticated = Authenticate(authToken);
XDocument xDoc = new XDocument();
XElement root = new XElement("Users");

[Code]....

I need to eliminate this way of generating xml for each table records.