Use XProcessingInstruction : XProcessingInstruction : XML LINQ C# Source Code

    
 
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Linq;
using System.Reflection;
using System.Xml.Linq;

class Program {
    static void Main(string[] args) {
        XNamespace w = "http://schemas.microsoft.com/office/word/2003/wordml";
        XDocument word = new XDocument(new XDeclaration("1.0", "utf-8", "yes"),
                                            new XProcessingInstruction("mso-application", "progid=\"Word.Document\""),
                                            new XElement(w + "wordDocument",
                                                new XAttribute(XNamespace.Xmlns + "w", w.NamespaceName)));
        System.IO.StringWriter sw = new System.IO.StringWriter();
        word.Save(sw);
        Console.WriteLine(sw);
    }
}