<%@ WebService Language="C#" Class="WebService" %>

public class WebService
{
    [System.Web.Services.WebMethod]
    public System.Xml.XmlDocument GetXml()
    {
        System.Xml.XmlDocument xml = new System.Xml.XmlDocument();
        xml.LoadXml("<XmlDoc>Test data</XmlDoc>");
        return xml;
    }
}

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {            
            localhost.WebService ws = new ConsoleApplication1.localhost.WebService();
            var xml = ws.GetXml();
            System.Console.WriteLine(xml.GetType());
            System.Console.ReadLine();
        }
    }
}