using System;

using System.IO;

using System.Text;

using System.Xml;

using System.Xml.Linq;

​

public class Program

{

    public static void Main()

    {

        string xml = @"<?xml version='1.0' encoding='utf-8'?>

            <Cooperations>

              <Cooperation />

            </Cooperations>";

        XDocument xDoc = XDocument.Parse(xml);

        Console.WriteLine(xDoc.ToStringWithDeclaration());

    }

}

​

public static class XDocumentExtensions

{

    public static string ToStringWithDeclaration(this XDocument doc)

    {

        if (doc == null)

        {

            throw new ArgumentNullException("doc");

        }

​

        StringBuilder builder = new StringBuilder();

        using (TextWriter writer = new StringWriter(builder))

        {

            doc.Save(writer);

        }

​

        return builder.ToString();

    }

}