Question

in the following code i am trying to handle different server resposes:
this function is used to read xml or html soruce from a web page
and in the commented line i get “Object reference not set to an instance of an object.”
i wonder why.

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

        Select Case CType(objResponse, Net.HttpWebResponse).StatusCode 'Here is where i get the error Object reference not set to an instance of an object.

    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

        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

now when i remove the switch statement and just write the objResponse as

objResponse = objRequest.GetResponse

except i get an exception of error 403 or 503, i don’t know how to handle this.

Answer 1

Your error line:

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

is called without ever assigning anything to objResponse.

You need to change it to:

objResponse = objRequest.GetResponse

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

In regards to getting HTTP error codes for the page you are trying to ftech, here are the meanings of them and their causes:

http://en.wikipedia.org/wiki/List_of_HTTP_status_codes