为啥我用FtpWebRequest 实现从FTP上获取文件列表是，提示报错550啊？
以下是我写的一段从FTP上获取文件列表的代码，当第一次调用的时候能正确获取，但第二次调用的时候就报错550，说远端文件不存在之类的，郁闷几天了，望哪位大侠指引一条明路，不胜感激！！！！！


'sFtpPath - FTP地址, sFtpUserID - FTP用户名,sFtpPassword - FTP密码, sLoadPath - FTP文件下载到本地的路径,nLen - 文件列表中文件的数量

'获取文件列表
Public Function GetFTPFile(ByVal sFtpPath As String, ByVal sFtpUserID As String, ByVal sFtpPassword As String, ByVal sLoadPath As String, ByVal nLen As Integer) As String()

Dim request As FtpWebRequest
Dim i As Integer = 0
Dim FileList(nLen) As String '文件列表

'On Error GoTo ErrorInformation

request = WebRequest.Create(sFtpPath)

request.Method = WebRequestMethods.Ftp.ListDirectory

request.UseBinary = False

request.Credentials = New NetworkCredential(sFtpUserID, sFtpPassword)

Dim response As FtpWebResponse = request.GetResponse

Dim responseStream As Stream = response.GetResponseStream()
Dim reader As StreamReader = New StreamReader(responseStream)

FileList(i) = reader.ReadLine
While Not IsNothing(FileList(i))
i = i + 1
FileList(i) = reader.ReadLine
End While

reader.Close()
response.Close()
responseStream.Close()

Return FileList
Exit Function

ErrorInformation:

MessageBox.Show("获取文件列表出错:" + Err.Description)
Exit Function

End Function

[解决办法]
检查ftp路径，防火墙或者权限
参考
http://www.cnblogs.com/lodestar/archive/2007/04/17/717110.html