using System;

using System.IO;

using System.Xml;

using System.Xml.Linq;

public class Program

{

    public static void Main()

    {

        var xmlExportFile = new XDocument(

            new XDeclaration("1.0", "UTF-8", "yes"),

            new XProcessingInstruction("qbxml", "version=\"2.0\""),

            new XElement("QBXML",

                         new XElement("QBXMLMsgsRq",

                                      new XAttribute("onError", "stopOnError")

                                     )

                        )

        );

        var xmlString = string.Empty;

        using (var stream = new MemoryStream())

        using (var writer = new StreamWriter(stream, System.Text.Encoding.UTF8))

        {

            xmlExportFile.Save(writer);

            xmlString = System.Text.Encoding.UTF8.GetString(stream.ToArray());

        }

        Console.WriteLine(xmlString);

    }

}