web 請求中的“try-catch”問題 vb.net

Question

我正在發出 web 請求，該請求大部分時間都成功完成（target$ 是一個 URL）。 但是偶爾我的代碼會拋出一個有效的異常，404 not found，如果 URL 目標 $ 不存在，並且執行停止。 編碼：

Sub scrape(target$)
    Dim request As WebRequest = WebRequest.Create(target$)
    Dim response As WebResponse = request.GetResponse()
    Dim dataStream As Stream = response.GetResponseStream()
    ' Open the stream using a StreamReader for easy access.
    Dim reader As New StreamReader(dataStream)
    ' Read the content.
    Dim responseFromServer As String = reader.ReadToEnd()
    txtResponse.Text = ""
    txtResponse.Text = responseFromServer
    ' Clean up the streams and the response.
    reader.Close()
    response.Close()

結束子

如果拋出異常，則會在第二行“Dim response...”中發生。 所以我嘗試添加一個“try-catch”，如圖所示。

Sub scrape(target$)
    Dim request As WebRequest = WebRequest.Create(target$)
    Dim response As WebResponse = request.GetResponse()
    Try
        Dim dataStream As Stream = response.GetResponseStream()
    Catch
        exflag = True
    End Try
    ' Open the stream using a StreamReader for easy access.
    Dim reader As New StreamReader(dataStream)
    ' Read the content.
    Dim responseFromServer As String = reader.ReadToEnd()
    txtResponse.Text = ""
    txtResponse.Text = responseFromServer
    ' Clean up the streams and the response.
    reader.Close()
    response.Close()

結束子

但是現在當我嘗試編譯代碼時，VisualStudio 告訴我“未聲明數據流”並且編譯失敗。

我做錯了什么以及如何在拋出異常時捕獲異常？

謝謝...