using System;

using System.Xml.Linq;

​

public class Program

{

    public static void Main()

    {

        const string xml =

@"<Root>

    <Child>Value</Child>

</Root>";

​

    var root = XElement.Parse(xml);

​

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

​

    foreach (var element in root.DescendantsAndSelf())

    {

        element.Name = ns + element.Name.LocalName;

    }

​

    root.Add(new XAttribute(XNamespace.Xmlns + "ex", ns));

​

        Console.WriteLine(root);

    }

}