Looks up the closest xmlns declaration for the given namespace URI
using System; using System.IO; using System.Xml; public class Sample { public static void Main() { XmlDocument doc = new XmlDocument(); doc.LoadXml("<book xmlns:bk='urn:samples' bk:ISBN='1-111111-57-5'>" + "<title>C#</title>" + "</book>"); XmlNode root = doc.FirstChild; string prefix = root.GetPrefixOfNamespace("urn:samples"); XmlElement elem = doc.CreateElement(prefix, "style", "urn:samples"); elem.InnerText = "hardcover"; root.AppendChild(elem); doc.Save(Console.Out); } }