Async XML write is clearing file

Question

I am working on a class that creates or opens an xml file:

namespace QTabs
{
    public class DataLayer
    {
        XmlDocument XMLDocObject = new XmlDocument();
        StorageFolder storageFolder = ApplicationData.Current.LocalFolder;
        StorageFile storageFile;
        string xmlFile = "DataStore.xml";
        public DataLayer()
        {
            loadXML();
        }

        public async void loadXML()
        {
            //if XML doc doesnt exist create it
            try
            {
                storageFile = await ApplicationData.Current.LocalFolder.GetFileAsync(xmlFile);
            }
            catch (FileNotFoundException)
            {
                storageFile = null;
            }
            if (storageFile == null)
            {

                storageFile = await storageFolder.CreateFileAsync(xmlFile);
                // Manipulate the xml document.
                var root = XMLDocObject.CreateElement("SyncTimes");
                XMLDocObject.AppendChild(root);
                var childTag1 = XMLDocObject.CreateElement("id");
                var childTag2 = XMLDocObject.CreateElement("dateTime");
                root.AppendChild(childTag1);
                root.AppendChild(childTag2);
                await XMLDocObject.SaveToFileAsync(storageFile);
            }
            else
            {
                storageFile = await storageFolder.GetFileAsync(xmlFile);
            }
        }
        public async void addSyncTime()
        {
            XmlElement x1 = XMLDocObject.CreateElement("SyncTime");
            XmlElement x11 = XMLDocObject.CreateElement("id");
            x11.InnerText = "1";
            x1.AppendChild(x11);
            XmlElement x12 = XMLDocObject.CreateElement("dateTime");
            x12.InnerText = DateTime.Now.ToString();
            x1.AppendChild(x12);
            await XMLDocObject.SaveToFileAsync(storageFile);
        }
    }
}

My issue is with the above addSyncTime() method. When it runs it results in the XML file being empty, including removing the elements added on creation.

What am I doing wrong?

Answer 1

Hard to say without a good, minimal, complete code example that reliably reproduces the problem. However, your code appears to have two different problems:

1. When the XML file is already present, you don't ever bother to actually load it, leaving XMLDocObject referencing an empty document. (Indeed, the storageFile != null case seems completely out of place; not only does it not load the XML from storageFile as I'd expect, it actually re-retrieves the storageFile object by calling GetFileAsync() again).
2. Your addSyncTime() method creates new elements, but never adds them to the XML document tree.

Given the above, I would expect the program behavior to be:

• The first time you run the program, the XML document is created.
• Any subsequent time you run the program (unless you delete the XML document), attempts to update the XML document will empty the document.

In neither case would I expect new SyncTime elements to be added. The first time you run the program, you'll get the root SyncTimes element and its initial id and dateTime children, but not any SyncTime elements. After that, the XML document will be emptied, and you'll still not get any new SyncTime elements.

If the above does not address your concern, please provide a good code example as described in the link I provide above.