web 请求中的“try-catch”问题 vb.net

Question

我正在发出 web 请求，该请求大部分时间都成功完成（target$ 是一个 URL）。 但是偶尔我的代码会抛出一个有效的异常，404 not found，如果 URL 目标 $ 不存在，并且执行停止。 编码：

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()

结束子

如果抛出异常，则会在第二行“Dim response...”中发生。 所以我尝试添加一个“try-catch”，如图所示。

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()

结束子

但是现在当我尝试编译代码时，VisualStudio 告诉我“未声明数据流”并且编译失败。

我做错了什么以及如何在抛出异常时捕获异常？

谢谢...