Question

Why this work like this:

        XDocument dataFeed = XDocument.Parse(e.Result);
        var guide = from query in dataFeed.Descendants("MaxPayne3")
                                    select new NewGamesClass
                                    {
                                        GameID = (string)query.Element("ID"),
                                        GameTitle = (string)query.Element("Title"),
                                        GameDescription = (string)query.Element("Description"),
                                        GameGuide = (string)query.Element("Guide")
                                    };

        if (NavigationContext.QueryString.TryGetValue("selectedItem", out selectedIndex))
        {
            if (selectedIndex == "0")
                GuidesListBox.ItemsSource = guide.Where(ngc => ngc.GameTitle.StartsWith("Feel"));
            else if (selectedIndex == "1")
                GuidesListBox.ItemsSource = guide.Where(ngc => ngc.GameTitle.StartsWith("Serious"));

But not like this:

            if (selectedIndex == "0")
                GuidesListBox.ItemsSource = guide.Where(ngc => ngc.GameID.StartsWith("000"));
            else if (selectedIndex == "1")
                GuidesListBox.ItemsSource = guide.Where(ngc => ngc.GameID.StartsWith("001"));

i want to use the GameID Instead The GameTitle.

Sorry about no XML, too early in the morning for me, lol

Here:
https://dl.dropbox.com/u/27136243/AchivementHunters/XML/GameList.xml

Here is the Class:

public class NewGamesClass
{
    string gameID;
    string gameTitle;
    string gamedescription;
    string gameImage;
    string gameGuide;
    string videoLink;
    public string GameID
    { get { return gameID; } set { gameID = value; } }
    public string GameTitle
    { get { return gameTitle; } set { gameTitle = value; } }
    public string GameDescription
    { get { return gamedescription; } set { gamedescription = value; } }
    public string GameImage
    { get { return gameImage; } set { gameImage = value; } }
    public string GameGuide
    { get { return gameGuide; } set { gameGuide = value; } }
    public string VideoLink
    { get { return videoLink; } set { videoLink = value; } }
}

Answer 1

Most of the <MaxPayne3> nodes do not have a child <ID> node, hence this code:

select new NewGamesClass
           {
               GameID = (string)query.Element("ID"),
               GameTitle = (string)query.Element("Title"),
               GameDescription = (string)query.Element("Description"),
               GameGuide = (string)query.Element("Guide")
            }

will produce mostly nulls for GameID. Then this code:

guide.Where(ngc => ngc.GameID.StartsWith("000"))

will throw NullReferenceException for all those elements

change it to this:

guide.Where(ngc => !String.IsNullOrEmpty(ngc.GameID) && ngc.GameID.StartsWith("000"))

and it should work fine.