Question

I am trying to make a download manager for my program. But when I run this code it gives me the message box say “Download Started” but that is it. I do not get any file downloaded or progress bar change? Does anyone know why?

Public Class frmDownloader

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        WebClient1.DownloadFileAsync(New Uri("https://s3.amazonaws.com/MinecraftDownload/launcher/Minecraft_Server.exe"), "C:\hi.exe")
        MsgBox("download started")
    End Sub

    Private Sub WebClient1_DownloadProgressChanged(ByVal sender As Object, ByVal e As System.Net.DownloadProgressChangedEventArgs) Handles WebClient1.DownloadProgressChanged
        ProgressBar1.Value = e.ProgressPercentage
        MsgBox("Download Progress Changed")
    End Sub


End Class

Answer 1

The DownloadProgressChanged method is invoked on a different thread than the one that started the download. Inside this callback you seem to be manipulating some GUI element: ProgressBar1.Value. You should never manipulate GUI elements on different threads than the one on which they were created or you might get an exception. Depending on the type of application you are working on there are different ways to marshal calls on the GUI thread. For example in WinForms you should use the Control.BeginInvoke method. In WPF and Silverlight the equivalent is the Dispatcher.BeginInvoke.