using System;

using System.Xml.Linq;

using System.Xml;

public class Program

{

    public static void Main()

    {

        var xml = "<?xml version=\"1.0\" encoding=\"utf-8\"?><foo><bar>Baz</bar></foo>";

        //Using older XmlDocument approach

        var doc = new XmlDocument();

        doc.LoadXml(xml);                               //Load up the original xml string into XmlDocument object

        var fooNode = doc.SelectSingleNode("//foo");    //use xpath to get to the top level foo element

        var newBar = doc.CreateElement("bar");          //Create a new bar element and assign it's inner text

        newBar.InnerText = "Baz2";

        fooNode.AppendChild(newBar);    //append the newly created bar element to foo

        Console.WriteLine(doc.InnerXml);        

        //Using XDocument

        var xdoc = XDocument.Parse(xml);                //Load up the original xml string into Linq XDocument object

        xdoc.Root.Add(new XElement("bar", "Bar2"));     //Add <bar>Bar2</bar> to foo (which is root of the document)

        Console.WriteLine(xdoc.ToString());

    }

}