为什么FTP下载的时候文件内容丢失

//附件下载事件
private void btnDownLoad_Click(object sender, EventArgs e)
{
if (lstFTP.SelectedItem != null)
{
string tFileName = lstFTP.SelectedItem.ToString();
int tIndex = tFileName.IndexOf(' ');
if (tIndex >= 0 && tFileName.Length > tFileName.Length - tIndex)
tFileName = tFileName.Substring(tIndex + 1, tFileName.Length - tIndex - 1);
tFileName = tFileName.Trim();
//string nUri = txtFtpUri.Text + "/" + tFileName;
string nUri = nUridz + tFileName;
DownloadProgressChangedEventHandler nProgressDlg = new DownloadProgressChangedEventHandler(client_DownloadProgressChanged);
AsyncCompletedEventHandler nCompletedDlg = new AsyncCompletedEventHandler(client_DownloadFileCompleted);

            SaveFileDialog sfd = new SaveFileDialog();
            sfd.FileName = tFileName;
            sfd.Filter = "所有文件(*.*)|*.*";
            sfd.ShowDialog();
            if (!sfd.FileName.Equals(string.Empty))
            {
                //gFtp.DownloadFileAsync(nUri, Path.Combine(_saveDir, tFileName), nProgressDlg, nCompletedDlg);
                gFtp.DownloadFileAsync(nUri, sfd.FileName, nProgressDlg, nCompletedDlg);
            }
        }
    }
    //下载完成
    public void client_DownloadFileCompleted(object sender, AsyncCompletedEventArgs e)
    {
        if (e.Error == null)
        {
            lblMessage.Text = "下载完成";
        }
    }
    //附件下载动画
    public void client_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)
    {
        this.proBarDownLoad.Minimum = 0;
        this.proBarDownLoad.Maximum = (int)gFtp.UriFileTotleSize;
        this.proBarDownLoad.Value = (int)e.BytesReceived;
        float f_total = gFtp.UriFileTotleSize;
        float f_value = e.BytesReceived;
        if (f_total <= 0)
            return;
        float tPercent = f_value / f_total * 100;
        this.lblercent_Down.Text = (int)tPercent + "%";
    }  //附件下载事件
    private void btnDownLoad_Click(object sender, EventArgs e)
    {
        if (lstFTP.SelectedItem != null)
        {
            string tFileName = lstFTP.SelectedItem.ToString();
            int tIndex = tFileName.IndexOf(' ');
            if (tIndex >= 0 && tFileName.Length > tFileName.Length - tIndex)
                tFileName = tFileName.Substring(tIndex + 1, tFileName.Length - tIndex - 1);
            tFileName = tFileName.Trim();
            //string nUri = txtFtpUri.Text + "/" + tFileName;
            string nUri = nUridz + tFileName;
            DownloadProgressChangedEventHandler nProgressDlg = new DownloadProgressChangedEventHandler(client_DownloadProgressChanged);
            AsyncCompletedEventHandler nCompletedDlg = new AsyncCompletedEventHandler(client_DownloadFileCompleted);

            SaveFileDialog sfd = new SaveFileDialog();
            sfd.FileName = tFileName;
            sfd.Filter = "所有文件(*.*)|*.*";
            sfd.ShowDialog();
            if (!sfd.FileName.Equals(string.Empty))
            {
                //gFtp.DownloadFileAsync(nUri, Path.Combine(_saveDir, tFileName), nProgressDlg, nCompletedDlg);
                gFtp.DownloadFileAsync(nUri, sfd.FileName, nProgressDlg, nCompletedDlg);
            }
        }
    }
    //下载完成
    public void client_DownloadFileCompleted(object sender, AsyncCompletedEventArgs e)
    {
        if (e.Error == null)
        {
            lblMessage.Text = "下载完成";
        }
    }
    //附件下载动画
    public void client_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)
    {
        this.proBarDownLoad.Minimum = 0;
        this.proBarDownLoad.Maximum = (int)gFtp.UriFileTotleSize;
        this.proBarDownLoad.Value = (int)e.BytesReceived;
        float f_total = gFtp.UriFileTotleSize;
        float f_value = e.BytesReceived;
        if (f_total <= 0)
            return;
        float tPercent = f_value / f_total * 100;
        this.lblercent_Down.Text = (int)tPercent + "%";
    }

gFtp是什么类？自己写的还是使用第三方的库？