using System;

using System.Xml;

using System.Xml.Linq;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var xml1 = @"<bookstore>

   <book>

     <bookID>100</bookID>

     <name> The cat in the hat </name>

   </book>

   <book>

    <bookID>90</bookID>

    <name> another book </name>

   </book>

   <book>

      <bookID>103</bookID>

      <name> a new book </name>

  </book>

</bookstore>";

            var xml2 = @"<bookstore>

  <book>

    <bookID>100</bookID>

    <content> story </content>

  </book>

  <book>

    <bookID>90</bookID>

    <content> fiction </content>

  </book>

  <book>

    <bookID>103</bookID>

    <content> bio </content>

  </book>

 </bookstore>";

        var fileone = XDocument.Parse(xml1);

        var filetwo = XDocument.Parse(xml2);

        var bookInfos =

            from a in fileone.Descendants("book")

            join b in filetwo.Descendants("book")

            on (string)a.Element("bookID") equals (string)b.Element("bookID")

            select new XElement("bookInfo", a.Element("bookID"), a.Element("name"), b.Element("content"));

        var result = new XElement("result", bookInfos);

        Console.WriteLine(result.ToString());

    }

}