Assembly: System (in System.dll)
'Declaration Public Delegate Sub DownloadDataCompletedEventHandler ( _ sender As Object, _ e As DownloadDataCompletedEventArgs _ )
When you create a delegate, you identify the method that will handle the event. To associate the event with your event handler, add an instance of the delegate to the event. The event handler is called whenever the event occurs, unless you remove the delegate. For more information about event handler delegates, see Events and Delegates.
The following code example demonstrates downloading a user-specified resource.
' Sample call : DownLoadDataInBackground ("http:' www.contoso.com/GameScores.html") Public Shared Sub DownloadDataInBackground(ByVal address As String) Dim waiter As System.Threading.AutoResetEvent = New System.Threading.AutoResetEvent(False) Dim client As WebClient = New WebClient() ' Specify that the DownloadDataCallback method gets called ' when the download completes. AddHandler client.DownloadDataCompleted, AddressOf DownloadDataCallback Dim uri as Uri = New Uri(address) client.DownloadDataAsync(uri, waiter) ' Block the main application thread. Real applications ' can perform other tasks while waiting for the download to complete. waiter.WaitOne() End Sub
The following method is called when the download completes.
Private Shared Sub DownloadDataCallback(ByVal sender As Object, ByVal e As DownloadDataCompletedEventArgs) Dim waiter As System.Threading.AutoResetEvent = CType(e.UserState, System.Threading.AutoResetEvent) Try ' If the request was not canceled and did not throw ' an exception, display the resource. If e.Cancelled = False AndAlso e.Error Is Nothing Then Dim data() As Byte = CType(e.Result, Byte()) Dim textData As String = System.Text.Encoding.UTF8.GetString(data) Console.WriteLine(textData) End If Finally ' Let the main application thread resume. waiter.Set() End Try End Sub