const string KEY = "MyUniqueKey";
 
private void SaveButton_Click(object sender, EventArgs e)
{
    var _Store = IsolatedStorageFile.GetUserStoreForApplication();
    using (var _Stream 
        = new IsolatedStorageFileStream(KEY, FileMode.Create, _Store))
    {
        var _Data = MyTextBox.Text;
        var _Serializer = new XmlSerializer(typeof(string));