Creating files and folders in WP8 - windows-phone-8

Question

I'm newbie on WP8 and trying to build up an app which reads constant variables from XML file and write user changes to another xml file.
However, I failed to create folders or files with following code.
public MainPage()
{
this.InitializeComponent();
this.NavigationCacheMode = NavigationCacheMode.Required;
this.writeFile();
}
public async void writeFile()
{
XDocument xDoc = XDocument.Load("Values.xml");
lblSafakCount.Text = xDoc.Descendants("isFirstUse").FirstOrDefault().Value.ToString();
string input = #"<isFirstUse>NO</isFirstUse>";
var replacement = XElement.Parse(input);;
byte[] bytes = System.Text.Encoding.UTF8.GetBytes(input);
StorageFolder local = ApplicationData.Current.LocalFolder;
if (local != null)
{
StorageFolder sf = await local.CreateFolderAsync("Res", CreationCollisionOption.ReplaceExisting);
StorageFile file = await sf.CreateFileAsync("val.xml", CreationCollisionOption.OpenIfExists);
using (var stream = await file.OpenStreamForWriteAsync())
{
stream.Seek(0, SeekOrigin.End);
stream.Write(bytes, 0, bytes.Length);
}
}
}
I've successfully read isFirstUse parameter from Values.xml so I have no problem with reading from files. When I try to examine codes step by step, app throws no error during the test but not creating folder or file.
Can you please help?
Thanks.

Answer 1

Here are my read and write based on your functions
If you want to write to the SD Card more information can be found here
Access the SD card in Windows Phone
public async void MyWriteFile()
{
string input = #"<isFirstUse>NO</isFirstUse>";
byte[] bytes = System.Text.Encoding.UTF8.GetBytes(input);
StorageFolder local = ApplicationData.Current.LocalFolder;
if (local != null)
{
StorageFolder sf = await local.CreateFolderAsync("Res", CreationCollisionOption.ReplaceExisting);
StorageFile file = await sf.CreateFileAsync("val.xml", CreationCollisionOption.OpenIfExists);
using (var stream = await file.OpenStreamForWriteAsync())
{
stream.Seek(0, SeekOrigin.End);
stream.Write(bytes, 0, bytes.Length);
}
}
}
private async void MyReadFile()
{
byte[] bytes = new byte[256];
StorageFolder local = ApplicationData.Current.LocalFolder;
if (local != null)
{
StorageFolder sf = await local.CreateFolderAsync("Res", CreationCollisionOption.OpenIfExists);
StorageFile file = await sf.CreateFileAsync("val.xml", CreationCollisionOption.OpenIfExists);
using (var stream = await file.OpenStreamForReadAsync())
{
stream.Read(bytes, 0, 256);
}
}
}