System.Net.FtpWebRequest.BeginGetRequestStream C# (CSharp) Method

        public override IAsyncResult BeginGetRequestStream(AsyncCallback callback, object state)
        {
            if (NetEventSource.IsEnabled)
            {
                NetEventSource.Enter(this);
                NetEventSource.Info(this, $"Method: {_methodInfo.Method}");
            }

            ContextAwareResult asyncResult = null;
            try
            {
                if (_getRequestStreamStarted)
                {
                    throw new InvalidOperationException(SR.net_repcall);
                }
                _getRequestStreamStarted = true;
                if (!_methodInfo.IsUpload)
                {
                    throw new ProtocolViolationException(SR.net_nouploadonget);
                }
                CheckError();

                FinishRequestStage(RequestStage.RequestStarted);
                asyncResult = new ContextAwareResult(true, true, this, state, callback);
                lock (asyncResult.StartPostingAsyncOp())
                {
                    _writeAsyncResult = asyncResult;
                    SubmitRequest(true);
                    asyncResult.FinishPostingAsyncOp();
                    FinishRequestStage(RequestStage.CheckForError);
                }
            }
            catch (Exception exception)
            {
                if (NetEventSource.IsEnabled) NetEventSource.Error(this, exception);
                throw;
            }
            finally
            {
                if (NetEventSource.IsEnabled) NetEventSource.Exit(this);
            }

            return asyncResult;
        }

 private void UploadNextFile(IAsyncResult ar)
 {
     //清空
     _currLength = 0;
     _abort = false;
     if (_uploadResponse != null) _uploadResponse.Close();
     if (_fileStream != null) _fileStream.Close();
     if (_requestStream != null) _requestStream.Close();
     _uploadResponse = (FtpWebResponse)_uploadRequest.EndGetResponse(ar);
     //_result = "文件上传成功";
     try
     {
         if (_uploadFiles.Count != 0)
         {
             FileInfo file = _uploadFiles[0] as FileInfo;
             ServerFileInfo serverFile = SearchFile(file.Name.Replace('#', '_'));
             if (serverFile != null) _currLength = serverFile.Size;
             else _currLength = 0;
             string uploadUrl = @"ftp://" + this.Address + ":" + this.Port + "/" + file.Name.Replace('#', '_');
             _uploadRequest = (FtpWebRequest)WebRequest.Create(uploadUrl);
             if ((bool)_uploadFilesStateTable[file])
             {//覆盖
                 _uploadRequest.Method = WebRequestMethods.Ftp.UploadFile;
                 _currLength = 0;
             }
             else _uploadRequest.Method = _currLength == 0 ? WebRequestMethods.Ftp.UploadFile : WebRequestMethods.Ftp.AppendFile; 
             _uploadRequest.Credentials = new NetworkCredential(this.UserName, this.Password);
             _uploadRequest.Proxy = null;
             _uploadRequest.KeepAlive = false;
             _uploadRequest.BeginGetRequestStream(new AsyncCallback(TransFile), file);
         }
     }
     catch (Exception e)
     {
         _result = e.Message;
     }
 }