using System;
using System.Collections.Generic;
using System.IO;
using System.Text;
using System.Xml;
namespace ConsoleApplication1
{
    class Program
    {
        public static void Main(string[] args)
        {
            var doc = new XmlDocument();
            var stream = new MemoryStream();

            XmlDeclaration xmlDeclaration = doc.CreateXmlDeclaration("1.0", "utf-8", null);
            XmlElement rootNode = doc.CreateElement("rexim");
            rootNode.InnerText = "давлоыдаывалдыовадлы";
            rootNode.SetAttribute("xmlns", "rexim:coding-team:test");
            doc.InsertBefore(xmlDeclaration, doc.DocumentElement);
            doc.AppendChild(rootNode);

            XmlWriterSettings settings = new XmlWriterSettings()
            {
                Indent = true
            };

            using( var writer = XmlWriter.Create(stream,settings)) {
                doc.Save(writer);
            }
        }
    }
}