在同一个循环中读取文件+下载网页？

Question

这是一个新手问题。

在循环中，我需要从文本文件中读取URL列表，下载网页并使用正则表达式搜索一些文本。

我用下面的代码，用DownloadStringAsync（），以避免冻结用户界面，但它触发的错误“Web客户端不支持并发I/O操作。”：

Private Sub AlertStringDownloaded(ByVal sender As Object, ByVal e As DownloadStringCompletedEventArgs) 
    If e.Cancelled = False AndAlso e.Error Is Nothing Then 
     Dim Content As String 
     Content = CStr(e.Result) 

     'Here, use regex to extract token 
    End If 
End Sub 

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click 
    'Read URLs from txt file 
    Dim FILE_NAME As String = "sites.txt" 

    Dim webClient As New WebClient 
    webClient.Encoding = System.Text.Encoding.UTF8 
    AddHandler webClient.DownloadStringCompleted, AddressOf AlertStringDownloaded 

    If System.IO.File.Exists(FILE_NAME) = True Then 
     Dim objReader As New System.IO.StreamReader(FILE_NAME) 

     Do While objReader.Peek() <> -1 
      Line = objReader.ReadLine 

      '"WebClient does not support concurrent I/O operations." 
      webClient.DownloadStringAsync(New Uri(Line)) 

      'Go to AlertStringDownloaded 
     Loop 
    Else 
     MsgBox("File Does Not Exist") 
    End If 

End Sub 

是否有人知道的替代？

谢谢。

---

编辑：显然，解决方案是强制应用程序暂停，直到当前页面被下载。有没有比使用DoEvents更好的方法？

Dim Busy As Boolean = False 

Private Sub AlertStringDownloaded(ByVal sender As Object, ByVal e As DownloadStringCompletedEventArgs) 
    If e.Cancelled = False AndAlso e.Error Is Nothing Then 
     ... 
     Busy = False 
    End If 
End Sub 

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click 
    ... 
    If System.IO.File.Exists(FILE_NAME) = True Then 
     Dim objReader As New System.IO.StreamReader(FILE_NAME) 

     Do While objReader.Peek() <> -1 
      ... 
      '"WebClient does not support concurrent I/O operations." 
      Busy = True 
      webClient.DownloadStringAsync(New Uri(Line)) 
      'Better way to pause until done downloading current page? 
      Do While Busy 
       System.Windows.Forms.Application.DoEvents() 
      Loop 
     Loop 
    Else 
    MsgBox("File Does Not Exist") 
    End If 
End Sub 

Answer 1

要创建一个同时执行两件事的程序，您应该查看线程。 （或者如果你有Visual Studio的下一个版本的CTP，那么Asynchrony）。请参阅blog post here for a simple tutorial on creating a multi-threaded app。