vb.net 尝试从某个页面读取 xml 时处理服务器响应答案

【问题标题】：vb.net Handling the server response when trying to read xml from some pagevb.net 尝试从某个页面读取 xml 时处理服务器响应
【发布时间】：2012-09-05 19:54:32
【问题描述】：

我有以下函数用于从我的网站获取 html 源代码

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

        Try
            ' Setup our Web request
            objRequest = Net.WebRequest.Create(URL)
            objRequest.Timeout = TimeoutSeconds * 1000
            ' Retrieve data from request

            Try
                objResponse = objRequest.GetResponse 'some times it gives an error server unavailable 503
            Catch ex As WebException
                MsgBox(ex.Message)
            End Try

            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
        Catch
            ' Error occured grabbing data, simply return nothing
            Return ""
        End Try
    End Function

有时 objResponse 会给出错误“503 服务器不可用”和许多其他错误，如 403 等，我该如何独立处理这些错误？

我怎样才能让这个函数在一段时间后重试请求？问题是 try 语句似乎没有处理这个问题，我不确定为什么我没有看到异常 MsgBox 但它在调试器上显示错误。

【问题讨论】：

标签： vb.net

【解决方案1】：

将响应转换为 HttpWebResponse 对象并对其 StatusCode 属性执行 Select Case。你必须清理并完成它，但这里有一个例子：

    Select Case CType(objResponse, Net.HttpWebResponse).StatusCode

        Case Net.HttpStatusCode.InternalServerError
            'This is sloppy, but a quick example for one of your sub-questions.
            System.Threading.Thread.Sleep(10000)
            'Try again.
            objResponse = objRequest.GetResponse
        Case Net.HttpStatusCode.BadRequest
            'Error Handling
        Case Net.HttpStatusCode.OK
            'Proceed as normal.
        Case Else
            'Error Handling

    End Select

【讨论】：