using System;

using System.Xml;

using System.Xml.Linq;

​

public class Program

{

    public static void Main()

    {

        const string xml =

            @"<!-- The comment -->

<Root xmlns=""http://www.namespace.com"">

    <FirstElement>

    </FirstElement>

​

    <SecondElement>

    </SecondElement>

</Root>";

​

        var doc = XDocument.Parse(xml, LoadOptions.PreserveWhitespace);

​

        XNamespace ns = "http://www.namespace.com";

​

        doc.Root.Add(

            new XText("\n\t"),

            new XElement(ns + "ThirdElement",

                new XText("\n\t\t"),

                new XElement(ns + "FourthElement", "thevalue"),

                new XText("\n\t")),

            new XText("\n"));

​

        Console.WriteLine(doc);

    }

}