Question

我想通过一个循环，但只有在抛出异常时才返回循环。我怎么写这个在vb.net？我正在尝试使用代理服务器发出Web请求。这是我到目前为止的代码，目前无效。

Public Sub checkproxy()

    Dim myWebRequest As WebRequest = WebRequest.Create("http://www.contoso.com")
    Dim myProxy As New WebProxy() 
    Dim proxyAddress = "http://www.example.com:8080"
    Dim newUri As New Uri(proxyAddress)
    myProxy.Address = newUri
    myWebRequest.Proxy = myProxy
    Dim switch As String = "b"
    Dim myWebResponse As WebResponse
    Do Until switch = "a"
        Try
            myWebRequest.GetResponse()
            switch = "a"
        Catch ex As Exception
            System.Threading.Thread.Sleep(5000)
        End Try
    Loop
    RichTextBox1.Text = myWebResponse.Headers.ToString
    myWebResponse.Close()
    ToolStripStatusLabel1.Text = "Connected"
End Sub

Answer 1

Exit Do正在寻找：

Do Until switch = "a"
    Try
        myWebRequest.GetResponse()
        switch = "a"
        Exit Do
    Catch ex As Exception
        System.Threading.Thread.Sleep(5000)
    End Try
Loop

Answer 2

实际上，您需要做的就是将Do Until ... Loop更改为Do ... Loop Until，它将始终运行循环一次，然后在再次执行循环之前检查循环条件：

Do 
    Try
        myWebRequest.GetResponse()
        switch = "a"
    Catch ex As Exception
        System.Threading.Thread.Sleep(5000)
    End Try
Loop Until switch = "a"

有关语法和进一步说明，请参阅Do Loop page

Answer 3

我需要{MarisJuraszek所述的Exit Do回答我还需要在try函数中创建webrequest而不是在代码的开头。这是工作代码的更新。

Public Sub checkproxy()

    Dim myWebRequest As WebRequest
    Dim myProxy As New WebProxy() 
    Dim proxyAddress = "http://www.example.com:8080"
    Dim newUri As New Uri(proxyAddress)
    myProxy.Address = newUri
    Dim switch As String = "b"
    Dim myWebResponse As WebResponse
    Do Until switch = "a"
        Try
            myWebRequest = WebRequest.Create("http://www.contoso.com")
            myWebRequest.Proxy = myProxy
            myWebResponse = myWebRequest.GetResponse()
            switch = "a"
            Exit Do
        Catch ex As Exception
            System.Threading.Thread.Sleep(5000)
        End Try
    Loop
    RichTextBox1.Text = myWebResponse.Headers.ToString
    myWebResponse.Close()
    ToolStripStatusLabel1.Text = "Connected"
End Sub

Answer 4

不需要此开关。

Do While True
    Try
        myWebRequest.GetResponse()
        Exit Do
    Catch ex As Exception
        System.Threading.Thread.Sleep(5000)
    End Try
Loop

此外，明智的做法是允许用户在设置的时间后退出循环，以避免无限循环。

循环直到没有例外

4 个答案: