using System;

using System.Linq;

using System.Xml;                   

using System.Xml.Linq;

using System.Collections; 

using System.Collections.Generic;

using System.Data;

using System.Data.DataSetExtensions;

using System.Data.Linq;

using System.Text;

using System.Text.RegularExpressions;

using System.Xml.Serialization; 

using System.IO;

using System.Globalization;

using System.Diagnostics;

public class Program

{

    public static void Main()

    {

        string input =@"<manifest xmlns:imsmd=""http://ltsc.ieee.org/xsd/LOM""><resources><resource><file href=""default.xml""><metadata><imsmd:lom /></metadata></file></resource></resources></manifest>";

        var doc = XDocument.Parse(input);

​

        doc.Descendants("resource")

            .ElementAt(0)

            .Add(new XElement("file", new XAttribute("href", "testimg.png"),

                 new XElement("metadata", new XElement(doc.Root.GetNamespaceOfPrefix("imsmd")+ "lom"))));

​

        doc.ToString().Dump();

    }

}