using System;

using System.Xml;

​

public class Program

{

    public static void Main()

    {

        var xml = @"<bookstore>

  <book>

    <bookID>100</bookID>

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

  </book>

  <book bookID='90'>

    <name> a new book </name>

  </book>

  <book>

    <bookID>103</bookID>

    <name> another book </name>

  </book>

</bookstore>";

        var doc = new XmlDocument();

        doc.LoadXml(xml);

        var result = doc.SelectNodes("//book/bookID/text() | //book/@bookID");

        foreach (XmlNode r in result)

        {

            Console.WriteLine(r.Value);

        }

    }

}