如何在BeginGetRequestStream上修复InvalidOperationException？

Question

我使用这种方法将文件上传到ftp：

public FtpUploader()
        {
            try
            {
                State state = new State();
                state.request = (FtpWebRequest)FtpWebRequest.Create(
                   ftpurl + "/" + Path.GetFileName(filename));
                state.request.Method = WebRequestMethods.Ftp.UploadFile;
                state.request.Credentials = new NetworkCredential(ftpusername, ftppassword);
                state.sourceStream = new StreamReader(filename);
                state.request.BeginGetRequestStream(new AsyncCallback(ConnectCallback), state);
                connectDone.WaitOne();

                state.fileContents = Encoding.UTF8.GetBytes(state.sourceStream.ReadToEnd());
                state.sourceStream.Close();

                Send(state);
                sendDone.WaitOne();
                state.requestStream.Close();

                Receive(state);
                receiveDone.WaitOne();

                Console.WriteLine("Upload File Complete, status {0}", state.stringResponse);

                state.response.Close();
            }
            catch (Exception err)
            {
                string t = err.ToString();
            }
        }

当它走到界限时：

Send(state);

然后它跳转到Send方法并尝试在内部执行该行：

private static void Send(State state)
        {
            state.request.BeginGetRequestStream(new AsyncCallback(SendCallback), state);
        }

当它尝试执行此行时，它会跳回到捕获区域的FtpUploader方法并抛出异常：

InvalidOperationException异常：

在前一个通话仍在进行中时，无法重新调用BeginGetRequestStream / BeginGetResponse

System.InvalidOperationException was caught
  HResult=-2146233079
  Message=Cannot re-call BeginGetRequestStream/BeginGetResponse while a previous call is still in progress.
  Source=System
  StackTrace:
       at System.Net.FtpWebRequest.BeginGetRequestStream(AsyncCallback callback, Object state)
       at mws.FtpUploader.Send(State state) in FtpUploader.cs:line 151
       at mws.FtpUploader..ctor() in FtpUploader.cs:line 63
  InnerException: 

第151行是：

state.request.BeginGetRequestStream(new AsyncCallback(SendCallback), state);

第63行是：

Send(state);