vb.net - 在 vb.net 中使用 select 语句处理 Net.WebResponse

Question

我有以下函数从某个页面获取 html 或 xml 源，当出现 403 异常时，我发现代码不会继续选择语句

   Public Function GetPageHTML(ByVal URL As String, _
        Optional ByVal TimeoutSeconds As Integer = 10) _
        As String
        ' Retrieves the HTML from the specified URL,
        ' using a default timeout of 10 seconds
        Dim objRequest As Net.WebRequest
        Dim objResponse As Net.WebResponse
        Dim objStreamReceive As System.IO.Stream
        Dim objEncoding As System.Text.Encoding
        Dim objStreamRead As System.IO.StreamReader


        ' Setup our Web request
        objRequest = Net.WebRequest.Create(URL)
        objRequest.Timeout = TimeoutSeconds * 5000
        ' Retrieve data from request

Try
        objResponse = objRequest.GetResponse 'some times it gives an error server unavailable 503

        Select Case CType(objResponse, Net.HttpWebResponse).StatusCode
            Case Net.HttpStatusCode.RequestTimeout
                Return "BadRequest"
            Case 403
                Return "BadRequest"
            Case Net.HttpStatusCode.GatewayTimeout
                Return "BadRequest"
            Case Net.HttpStatusCode.InternalServerError
                Return "BadRequest"
            Case Net.HttpStatusCode.BadRequest
                Return "BadRequest"
            Case Net.HttpStatusCode.OK

                objStreamReceive = objResponse.GetResponseStream
                objEncoding = System.Text.Encoding.GetEncoding( _
                    "utf-8")

                objStreamRead = New System.IO.StreamReader( _
                    objStreamReceive, objEncoding)
                ' Set function return value

                GetPageHTML = objStreamRead.ReadToEnd()
                ' Check if available, then close response
                If Not objResponse Is Nothing Then
                    objResponse.Close()
                End If
            Case Else
                MsgBox("Case Else")

        End Select
catch
Return ""
End Try
    End Function

那么我该如何解决呢？调试器在这一行显示异常

objResponse = objRequest.GetResponse 'some times it gives an error server unavailable 503 or 403

所以它不会进入选择语句

也第二次尝试同样的问题，它没有捕获错误并且没有显示异常消息