Visual Basic Console Application Download Percentage? - vb.net

Question

I am using DownloadFileAsync to download a larger file (1.3 GB), but i'd like to add a simple percentage indicator (ex. 64%). I'm new to Visual Basic I have no idea how to do this.
Any help would be appreciated.

Answer 1

The WebClient class has a DownloadProgressChanged event that you can listen to if you want to update a progresss display. For instance, if you’ve got a console application, it’s as simple as:
Dim client As New WebClient()
AddHandler client.DownloadProgressChanged, AddressOf ProgressUpdate
client.DownloadFileAsync(yourURI, yourFile)
Sub ProgressUpdate(sender As Object, e As DownloadProgressChangedEventArgs)
' Reset cursor position …
Console.CursorTop -= 1
Console.CursorLeft = 0
Console.WriteLine("{0}% completed", e.ProgressPercentage)
End Sub
If, on the other hand, you are on a Form in a WinForms project and you’ve got a label ProgressLabel that you want to update, the following code will do that:
Sub ProgressUpdate(sender As Object, e As DownloadProgressChangedEventArgs)
Dim s = String.Format("{0}% completed", e.ProgressPercentage)
Me.Invoke(New Action(Sub()
ProgressLabel.Text = s
End Sub))
End Sub
The ProgressUpdate method is a bit complicated due to multithreading:
The WebClient is running the asynchronous file download in a background thread. However, form controls can only be updated from the foreground thread that the form is running in. For that reason, we cannot update the label directly inside the ProgressUpdate event (because that, too, is being invoked, and running, in the background thread1).
So what we do instead is use the Form.Invoke method which guarantees that whatever we want to execute is execute in the form’s own thread. We pass an Action delegate to the Invoke method which contains the code that we want to execute. And that code is just updating the label.
1 At least I couldn’t find anything in the documentation saying otherwise – the event might actually execute in the foreground thread but in that case the above code still works.