using System;

using System.Xml;

using System.Xml.Linq;

using System.Xml.XPath;

public class Program

{

    public static void Main()

    {

            string str =

@"<?xml version=""1.0""?>  

<!-- comment at the root level -->  

<Root>  

    <RootChild1>

        <Child>Content</Child>  

        <Child>Content</Child>  

        <Child>Content</Child>  

    </RootChild1>

    <RootChild2>

        <Child>Content</Child>  

        <Child key='target'>Found me</Child>  

        <Child>Content</Child>  

    </RootChild2>

</Root>";

            XDocument doc = XDocument.Parse(str);

​

            foreach (XElement element in doc.Descendants("RootChild2"))

            {

                string result = element.XPathSelectElement("//RootChild2/Child[@key='target']").Value;

                Console.WriteLine(result);

            }

    }

}