在添加后台传输请求之前检查文件大小

Question

我是新手，我正在开发后台传输功能来下载文件。它适用于Windows Phone 7.但问题是超过100 MB的文件无法使用“Transfer Preference = Allow Battery”下载，如果我使用“Transfer Preference = None”，则必须将手机连接到电源才能传输（任何文件大小）。到目前为止，这是我尝试过的，但它只适用于超过100MB的文件。任何帮助或建议都会很棒。谢谢！

    private void downloadTrigger()
    {
        string transferFileName = urlTextBox.Text;
        var transferUri = new Uri(Uri.EscapeUriString(transferFileName), UriKind.Absolute);

        BackgroundTransferRequest transferRequest = new BackgroundTransferRequest(transferUri);
        transferRequest.Method = "GET";

        string downloadFile = transferFileName.Substring(transferFileName.LastIndexOf("/") + 1);
        Uri downloadUri = new Uri("shared/transfers/" + downloadFile, UriKind.Relative);
        transferRequest.DownloadLocation = downloadUri;
        transferRequest.Tag = downloadFile;

        if (transferRequest.TotalBytesToReceive >= 104857600)
        {
            try
            {
                transferRequest.TransferPreferences = TransferPreferences.None;
                MessageBox.Show("For files over 100MB an external power is required to start copy.", "News box", MessageBoxButton.OK);
                BackgroundTransferService.Add(transferRequest);
                feedbackTextBlock.Text = "Queueing " + downloadFile;
                return;
            }
            catch
            {
            }
        }

        try
        {
            transferRequest.TransferPreferences = TransferPreferences.AllowBattery;
            BackgroundTransferService.Add(transferRequest);
            feedbackTextBlock.Text = "Copying " + downloadFile;
        }
        catch
        {
        }
    }