VB.NET DownloadDataAsync:

I a开发者_C百科m having the worst trouble getting around a bug, and am hoping that I can get some advice on this site. In short, I am trying to make an asynchronous web service call from my VB.NET application. But my "client_DownloadDataCompleted" callback is NEVER being called when the download is complete.

Here is my complete code:

Public Sub BeginAsyncDownload(ByVal Url As String)

    Dim waiter As System.Threading.AutoResetEvent = New System.Threading.AutoResetEvent(False)
    Dim client As WebClient = New WebClient()

    'client_DownloadDataCompleted method gets called when the download completes.
    AddHandler client.DownloadDataCompleted, AddressOf client_DownloadDataCompleted

    Dim uri As Uri = New Uri(Url)
    Downloading = True  'Class variable defined elsewhere
    client.DownloadDataAsync(uri, waiter)

End Sub

Private Sub client_DownloadDataCompleted(ByVal sender As Object, ByVal e As AsyncCompletedEventArgs)
    MessageBox.Show("Download Completed")
    Downloading = False
    Debug.Print("Downloaded")
End Sub

Again, the client_DownloadDataCompleted method is never being called. I have also tried using the method:

Private Sub client_DownloadDataCompleted(ByVal sender As Object, ByVal e As DownloadDataCompletedEventArgs)

With no luck. What I really need is that "Downloading" variable to be switched off when the download is complete.

Thanks in advance! Brett

The client (Webclient) should be declared outside the BeginAsyncDownload subroutine, so it has a form/class level visibility. Please refer to the following code:

Public Class Form1
    Dim client as New WebClient()
    Private Sub BeginAsyncDownload(ByVal Url As String)  
       AddHandler client.DownloadDataCompleted, AddressOf client_DownloadDataCompleted                    

       Dim uri As Uri = New Uri(Url)                    
       Downloading = True  'Class variable defined elsewhere                    
       client.DownloadDataAsync(uri, waiter)   
    End Sub

Private Sub client_DownloadStringCompleted(ByVal sender As Object, ByVal e As System.Net.DownloadStringCompletedEventArgs)
   MessageBox.Show("Download Completed")                      
   Downloading = False                      
   Debug.Print("Downloaded")              
End Sub

This is a tough one. I spent a little time on this and wasn't able to figure out why it wasn't getting called, sorry.

If you aren't able to get this to work, I have some code on CodePlex that includes a WebHelper class that might help you. I tried to make it as easy to use as WebClient but with all the power of HttpWebRequest.

The project is called BizArk. I wrote it just as a repository of code for myself. Feel free to just use the bits you want, I don't have any particular interest in how the code is used (as long as it's not used for evil :).

In what context are you invoking the webclient? WebClient will pick up your SynchronizationContext.Current and post its completion callback to it.

If you are using WinForms and your UI thread is blocked it will never be able to process your callback.