c# 上传下载，基础对象已关闭~~~

 上传代码如下：
public bool fileUpload(string localPath, string ftpFileName)
        {
            bool success = false;
            FtpWebRequest ftpWebRequest = null;
            FileStream localFileStream = null;
            Stream requestStream = null;
            //FileStream fs = null;
            try
            {
                FileInfo localFile = new FileInfo(localPath);//检查本地是否存在要上传的文件
                if (localFile.Exists)
                {
    
                    string uri = ftpRootURL + ftpFileName;
                    ftpWebRequest = (FtpWebRequest)FtpWebRequest.Create(new Uri(uri));
                    ftpWebRequest.Credentials = new NetworkCredential(ftpUser, ftpPassword);
                    ftpWebRequest.UseBinary = true;
                    ftpWebRequest.KeepAlive = false;
                    ftpWebRequest.Method = WebRequestMethods.Ftp.UploadFile;
                    ftpWebRequest.ContentLength = localFile.Length;
                    int buffLength = 2048;
                    byte[] buff = new byte[buffLength];
                    int contentLen;
                    localFileStream = localFile.OpenRead();
                    requestStream = ftpWebRequest.GetRequestStream();    //报错处
                    contentLen = localFileStream.Read(buff, 0, buffLength);
                    while (contentLen != 0)
                    {
                        requestStream.Write(buff, 0, contentLen);
                        contentLen = localFileStream.Read(buff, 0, buffLength);
                    }
                    success = true;
                }
                else
                {
                    success = false;
                }
            }
            catch (Exception)
            {
                success = false;
            }
            finally
            {
                if (requestStream != null)
                {
                    requestStream.Close();
                    //fs.Close();
                }
                if (localFileStream != null)
                {
                    localFileStream.Close();
                }
                //if (ftpWebRequest != null)
                //{
                //    ftpWebRequest.();
                //}
            }
           
            return success;
        }
requestStream = ftpWebRequest.GetRequestStream(); 这里总是报错。我是有很多图片一起上传的，就是一个循环。有些能传上去有些不能，偶尔全部都能。报错是基础对象已关闭