防止在 Try 中未分配值的变量出现警告

Question

我在互联网上找到了一些代码如下（稍作修改）。

它只是请求网页的内容。

Private Sub readWebpage(ByVal url As String)
    Dim Str As System.IO.Stream
    Dim srRead As System.IO.StreamReader
    Try
        ' make a Web request
        Dim req As System.Net.WebRequest = System.Net.WebRequest.Create(url)
        Dim resp As System.Net.WebResponse = req.GetResponse
        Str = resp.GetResponseStream
        srRead = New System.IO.StreamReader(Str)
        ' read all the text 
        textContent.text = srRead.ReadToEnd
    Catch ex As Exception
        MsgBox(ex.Message, MsgBoxStyle.Critical, "Unable to download content from: " & url)
    Finally
        srRead.Close()
        Str.Close()
    End Try
End Sub

但是我收到两个警告：

Warning 1   Variable 'srRead' is used before it has been assigned a value. A null reference exception could result at runtime.


Warning 2   Variable 'Str' is used before it has been assigned a value. A null reference exception could result at runtime.

我知道我可以简单地忘记 Finally 并将代码添加到 try 块中。

这是可行的方法还是我可以使用不同的方法来防止警告？

预先感谢您启发我！ :)

Answer 1

默认情况下，您可以简单地将它们设置为 Nothing，这样您就可以通知编译器您知道自己在做什么:)

Dim Str As System.IO.Stream = Nothing

Answer 2

该警告是因为如果 GetResponseStream 出现错误，那么您的 srRead 将为 null，从而导致 Null 异常。

处理这个问题的一种方法是使用Using，它会自动处理这些对象

 Private Sub readWebpage(ByVal url As String)
        Try
            ' make a Web request
            Dim req As System.Net.WebRequest = System.Net.WebRequest.Create(url)
            Dim resp As System.Net.WebResponse = req.GetResponse
            Using Str As System.IO.Stream = resp.GetResponseStream
                Using srRead As System.IO.StreamReader = New System.IO.StreamReader(Str)
                    textContent = srRead.ReadToEnd
                End Using
            End Using


    Catch ex As Exception
        MsgBox(ex.Message, MsgBoxStyle.Critical, "Unable to download content from: " & url)

    End Try
  End Sub

你也可以按照Dr. Evil建议的方式将对象设置为Nothing而不是Using关键字，然后你会希望在你的finally中使用它

Finally 
        If Str Is Not Nothing Then
            Str.Close
        End If
        If srRead Is Not Nothing Then
            srRead.Close
        End If
End Try