How to read all child nodes of an XML file

I've got this file:

<?xml version='1.0' encoding='UTF-8'?>
<feed xmlns='http://www.w3.org/2005/Atom' xmlns:app='http://purl.org/atom/app#' xmlns:media='http://search.yahoo.com/mrss/' xmlns:openSearch='http://a9.com/-/spec/opensearchrss/1.0/' xmlns:gd='http://schemas.google.com/g/2005' xmlns:gml='http://www.opengis.net/gml' xmlns:yt='http://gdata.youtube.com/schemas/2007' xmlns:georss='http://www.georss.org/georss'>
    <id>http://gdata.youtube.com/feeds/api/users/snifyy/favorites</id>
    <updated>2011-09-26T16:21:40.933Z</updated>
    <开发者_运维技巧category scheme='http://schemas.google.com/g/2005#kind' term='http://gdata.youtube.com/schemas/2007#video'/>
    <title type='text'>Favorites of snifyy</title>
    <logo>http://www.youtube.com/img/pic_youtubelogo_123x63.gif</logo>
    <author>
        <name>snifyy</name>
        <uri>http://gdata.youtube.com/feeds/api/users/snifyy</uri>
    </author>
    <generator version='2.1' uri='http://gdata.youtube.com'>YouTube data API</generator>
    <openSearch:totalResults>631</openSearch:totalResults>
    <openSearch:startIndex>1</openSearch:startIndex>
    <openSearch:itemsPerPage>25</openSearch:itemsPerPage>
    <entry>
        <id>http://gdata.youtube.com/feeds/api/videos/LXO-jKksQkM</id>
        <published>2011-09-23T16:15:27.000Z</published>
        <updated>2011-09-26T16:21:15.000Z</updated>
        <title type='text'>PUMPED UP KICKS|DUBSTEP</title>
        <content type='text'>DUBSTEPPIN!!! to a beast track remixed by "butch clancy"</content>
        <link rel='alternate' type='text/html' href='http://www.youtube.com/watch?v=LXO-jKksQkM&amp;feature=youtube_gdata'/>
        <link rel='http://gdata.youtube.com/schemas/2007#video.responses' type='application/atom+xml' href='http://gdata.youtube.com/feeds/api/videos/LXO-jKksQkM/responses'/>
        <link rel='http://gdata.youtube.com/schemas/2007#video.related' type='application/atom+xml' href='http://gdata.youtube.com/feeds/api/videos/LXO-jKksQkM/related'/>
        <link rel='http://gdata.youtube.com/schemas/2007#mobile' type='text/html' href='http://m.youtube.com/details?v=LXO-jKksQkM'/>
        <link rel='related' type='application/atom+xml' href='http://gdata.youtube.com/feeds/api/videos/LXO-jKksQkM'/>
        <link rel='self' type='application/atom+xml' href='http://gdata.youtube.com/feeds/api/users/snifyy/favorites/LXO-jKksQkM'/>
        <author>
            <name>WHZGUD2</name>
            <uri>http://gdata.youtube.com/feeds/api/users/whzgud2</uri>
        </author>
        <gd:comments>
            <gd:feedLink href='http://gdata.youtube.com/feeds/api/videos/LXO-jKksQkM/comments' countHint='3738'/>
        </gd:comments>
        <media:group>
            <media:category label='Entertainment' scheme='http://gdata.youtube.com/schemas/2007/categories.cat'>Entertainment</media:category>
            <media:content url='http://www.youtube.com/v/LXO-jKksQkM?f=user_favorites&amp;app=youtube_gdata' type='application/x-shockwave-flash' medium='video' isDefault='true' expression='full' duration='327' yt:format='5'/>
            <media:content url='rtsp://v3.cache5.c.youtube.com/CioLENy73wIaIQlDQiypjL5zLRMYDSANFEgGUg51c2VyX2Zhdm9yaXRlcww=/0/0/0/video.3gp' type='video/3gpp' medium='video' expression='full' duration='327' yt:format='1'/>
            <media:content url='rtsp://v1.cache2.c.youtube.com/CioLENy73wIaIQlDQiypjL5zLRMYESARFEgGUg51c2VyX2Zhdm9yaXRlcww=/0/0/0/video.3gp' type='video/3gpp' medium='video' expression='full' duration='327' yt:format='6'/>
            <media:description type='plain'>DUBSTEPPIN!!! to a beast track remixed by "butch clancy"</media:description>
            <media:keywords>DUBSTEP, butch, clancy, bass</media:keywords>
            <media:player url='http://www.youtube.com/watch?v=LXO-jKksQkM&amp;feature=youtube_gdata_player'/>
            <media:thumbnail url='http://i.ytimg.com/vi/LXO-jKksQkM/0.jpg' height='360' width='480' time='00:02:43.500'/>
            <media:thumbnail url='http://i.ytimg.com/vi/LXO-jKksQkM/1.jpg' height='90' width='120' time='00:01:21.750'/>
            <media:thumbnail url='http://i.ytimg.com/vi/LXO-jKksQkM/2.jpg' height='90' width='120' time='00:02:43.500'/>
            <media:thumbnail url='http://i.ytimg.com/vi/LXO-jKksQkM/3.jpg' height='90' width='120' time='00:04:05.250'/>
            <media:title type='plain'>PUMPED UP KICKS|DUBSTEP</media:title>
            <yt:duration seconds='327'/>
        </media:group>
        <gd:rating average='4.96415' max='5' min='1' numRaters='24435' rel='http://schemas.google.com/g/2005#overall'/>
        <yt:statistics favoriteCount='16660' viewCount='924793'/>
    </entry>
</feed>

And I want to load all entries and then their title, content and thumbnail. But I have problems even loading the entries. That's my code (data is this xml):

    XmlDocument xml = new XmlDocument();
    xml.LoadXml(data);

    XmlNodeList xnList = xml.SelectNodes("feed/entry");
    foreach (XmlNode xn in xnList) {
        Debug.WriteLine(xn.LocalName.ToString());
    }

The problem is that your XPath doesn't specify the namespaces (so you're actually searching for names with null namespace URIs). Given that feed is the top level element, it's a pretty trivial query - I don't think using XPath is actually helping you here. Try this instead, using XmlElement.GetElementsByTagName:

XmlElement root = doc.DocumentElement;
String atomUri = "http://www.w3.org/2005/Atom";
foreach (XmlElement element in root.GetElementsByTagName("entry", atomUri))
{
    // Use element here
}

(If you get the chance to use .NET 3.5, I suggest you start using LINQ to XML - it's much nicer :)

EDIT: Here's a short but complete program printing out the url attribute of each content element:

using System;
using System.Xml;

class Program
{
    static void Main(string[] args)
    {
        XmlDocument doc = new XmlDocument();
        doc.Load("test.xml");

        XmlElement root = doc.DocumentElement;
        String atomUri = "http://www.w3.org/2005/Atom";
        String mediaUri = "http://search.yahoo.com/mrss/";
        foreach (XmlElement entry in root.GetElementsByTagName("entry", atomUri))
        {
            foreach (XmlElement group in 
                     entry.GetElementsByTagName("group", mediaUri))
            {
                foreach (XmlElement content in 
                         entry.GetElementsByTagName("content", mediaUri))
                {
                    Console.WriteLine(content.Attributes["url"].Value);
                }
            }
        }
    }
}

You can use XmlNamespaceManager to specify namspace from your feed:

XmlDocument xDoc = new XmlDocument();
xDoc.Load(data);

XmlNamespaceManager manager = new XmlNamespaceManager(xDoc.NameTable);
manager.AddNamespace("atom", "http://www.w3.org/2005/Atom");

XmlNodeList xnList = xDoc.SelectNodes("atom:feed/atom:entry", manager);
foreach (XmlNode xn in xnList)
{
    Debug.WriteLine(xn.LocalName.ToString());
}

And any type when you trying to select nodes use prefix of the namespace, eg: atom:feed/atom:entry

Adding another namespace to work with multiple ones:

XmlNamespaceManager manager = new XmlNamespaceManager(xDoc.NameTable);
manager.AddNamespace("atom", "http://www.w3.org/2005/Atom");
manager.AddNamespace("media", "http://search.yahoo.com/mrss/");

XmlNodeList mediaNodes = xDoc.SelectNodes("atom:feed/atom:entry/media:group", manager);

You need to use the Atom namespace when you select the feed/entry elements.

Create a namespace manager, add a namespace prefix for the Atom namespace, and then use it in your XPath expression.

For example code, see the accepted answer to this question.

Here's some help - http://www.kirupa.com/forum/showthread.php?292473-Reading-Child-nodes-from-XML-file-C

And How to read/write nodes and child nodes from xml file in c# maybe using xpath?