Add new XElement to Xdocument

Question

I have the following code, which successfully writes to an XML file. However, it overwrites each time because of the tagRegistry.Save() call being made. How can I add a new XElement to the existing file? At the moment the file is simply overwritten.

public void saveTag()
{
    if (File.Exists("/tagRegistry.xml"))
    {
        XElement tagRegistry = XElement.Load("/tagRegistry.xml");
        XElement newTag = new XElement("Tag",
        new XElement("tag", stringUid),
        new XElement("name", desiredName),
        new XElement("latitude", latitude),
        new XElement("longitude", longitude));
        tagRegistry.Add(newTag);

        using (var storage = IsolatedStorageFile.GetUserStoreForApplication())
        {
            using (Stream stream = storage.CreateFile("/tagRegistry.xml"))
            {
                tagRegistry.Save(stream);
            }
        }

    }
    else
    {
        XDocument tagRegistry = new XDocument(new XElement("SmartSafe"));
        tagRegistry.Element("SmartSafe").Add(new XElement("Tag",
                    new XElement("tag", stringUid),
                    new XElement("name", desiredName),
                    new XElement("latitude", latitude),
                    new XElement("longitude", longitude)));
        using (var storage = IsolatedStorageFile.GetUserStoreForApplication())
        {
            using (Stream stream = storage.CreateFile("/tagRegistry.xml"))
            {
                tagRegistry.Save(stream);
            }
        }
    }
}

Answer 1

It's possible your File.Exists call is wrong. You are storing the file to isolated storage, but reading in from your current running directory. So you're always falling into the else block and writing a new file every time.

Answer 2

Try this:

public void saveTag()
{
    using (var storage = IsolatedStorageFile.GetUserStoreForApplication())
    {
        XDocument document;
        XElement tagRegistry = null;

        if (storage.FileExists("/tagRegistry.xml"))
        {
            using(var stream = storage.OpenFile("/tagRegistry.xml", FileMode.Open))
            {
                document = XDocument.Load(stream);
            }

            tagRegistry = document.Descendants("SmartSafe").FirstOrDefault();
        }
        else
        {
            document = new XDocument();
        }

        if(tagRegistry == null)
        {
            tagRegistry = new XElement("SmartSafe");
            document.Add(tagRegistry);
        }

        XElement newTag = new XElement("Tag",
            new XElement("tag", stringUid),
            new XElement("name", desiredName),
            new XElement("latitude", latitude),
            new XElement("longitude", longitude));

        tagRegistry.Add(newTag);

        using (Stream stream = storage.CreateFile("/tagRegistry.xml"))
        {
            document.Save(stream);
        }
    }
}