[Invoke]
        public byte[] LoadFile(int attachmentID)
        {
            List<byte> bytes = new List<byte>();
            var query = (from attachment in DataContext.Attachments
                         where attachment.AttachmentID == attachmentID
                         select attachment).FirstOrDefault();
            if (query == null || query.Data == null)
                return bytes.ToArray();
            return query.Data.ToArray();
        }

  private void button1_Click(object sender, RoutedEventArgs e)
        {
            _saveDialog = new SaveFileDialog();
            bool? result = _saveDialog.ShowDialog();
            if (result == true)
                _dbDomainContext.LoadFile(40, UploadCompleted, null);
        }

        private void UploadCompleted(InvokeOperation args)
        {
            byte[] file = (byte[]) args.Value;
            _saveFileStream = _saveDialog.OpenFile();
            StreamWriter sw = new StreamWriter(_saveFileStream);
            sw.BaseStream.Write(file, 0, file.Length);
            sw.Flush();
            sw.Close();
        }