将文件从Web下载到PC中的特定位置

Question

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    Dim imageAddress As String = String.Empty
    Dim filename As String = String.Empty

    Try
        Dim fileReader As New WebClient()

        Dim proxy As IWebProxy = New WebProxy("proxy adress", port number)
        proxy.Credentials = New NetworkCredential("network userid", "network password")

        imageAddress = "http://wwwimages.adobe.com/content/dam/Adobe/en/products/acrobat/pdfs/adobe-acrobat-xi-esign-pdf-file-tutorial-ue.pdf"

        filename = imageAddress.Substring(imageAddress.LastIndexOf("/") + 1)

        If Not (System.IO.File.Exists("D:\" + filename)) Then
            fileReader.DownloadFile(imageAddress, "D:\" + filename)
        End If
    Catch ex As HttpListenerException
        MsgBox("Error accessing " + imageAddress + " - " + ex.Message)
    Catch ex As Exception
        MsgBox("Error accessing " + imageAddress + " - " + ex.Message)
   End Try
End Sub

以上代码抛出错误407：需要代理身份验证。我似乎不理解我的代码的问题。任何帮助表示赞赏。谢谢提前