关于FtpWebRequest有关问题

关于FtpWebRequest问题
我写了两个方法a和b，先后调用Download方法，a方法先执行，b方法后执行，每次b方法调用Download方法时抛出“文件不可用”的异常。两个方法传的par_downloadUrl参数都是对的。谁能帮忙解决一下，先谢谢了。

 Public Sub Download(ByVal par_downloadUrl As String)
  Dim responseStream As Stream = Nothing
  Dim fileStream As FileStream = Nothing
  Dim reader As StreamReader = Nothing
  Try
  Dim downloadRequest As FtpWebRequest = WebRequest.Create(par_downloadUrl)
  downloadRequest.Credentials = New NetworkCredential("", "")
  Dim downloadResponse As FtpWebResponse = downloadRequest.GetResponse()
  responseStream = downloadResponse.GetResponseStream()
  Dim fileName As String = Path.GetFileName(downloadRequest.RequestUri.AbsolutePath)
  If fileName.Length = 0 Then
  reader = New StreamReader(responseStream)
  Else
  fileStream = File.Create("C://" & fileName)
  Dim buffer(GetFileSize(par_downloadUrl)) As Byte

  Dim bytesRead As Integer
  While True
  bytesRead = responseStream.Read(buffer, 0, buffer.Length)
  If bytesRead = 0 Then
  Exit While
  End If
  fileStream.Write(buffer, 0, bytesRead)
  End While

  End If
  Catch ex As UriFormatException
  MessageBox.Show(ex.Message)
  Catch ex As WebException
  MessageBox.Show("1" & ex.Message)
  Catch ex As IOException
  MessageBox.Show(ex.Message)
  Finally
  If reader IsNot Nothing Then
  reader.Close()
  ElseIf responseStream IsNot Nothing Then
  responseStream.Close()
  End If
  If fileStream IsNot Nothing Then
  fileStream.Close()
  End If

  End Try
  End Sub

------解决方案--------------------
试试加上downloadRequest.keepalive = false这句。