using System;

using System.Xml;

using System.IO;

using System.Text;

public class Program

{

    public static void Main()

    {

        var document = new XmlDocument();

        document.CreateXmlDeclaration("1.0", "utf-8", null);

​

        var foo = document.CreateElement("foo");

        document.AppendChild(foo);

        foo.SetAttribute("xmlns:xsi", "http://www.w3.org/2001/XMLSchema-instance");

​

        var bar = document.CreateElement("bar");

        bar.SetAttribute("nil", "http://www.w3.org/2001/XMLSchema-instance", "true");

        foo.AppendChild(bar);

​

        using var stream = new MemoryStream();

        using var xmlWriter = XmlWriter.Create(stream, new XmlWriterSettings() { OmitXmlDeclaration = false, Indent = true });

        document.Save(xmlWriter);

        string result = Encoding.UTF8.GetString(stream.ToArray());

        Console.WriteLine(result);

    }

}