[RESOLVED] Insert pop up error if connection drops or download fails

[jmcilhinney]

I wasn't suggesting that you add a Try...Catch block to your DownloadFileCompleted event handler. The exception handler is built into WebClient class. The WebClient tries to download the file and, if an exception is thrown, it catches it. Once the download is complete, either successfully or unsuccessfully, the DownloadFileCompleted event is raised. In the event handler, you check whether an exception occurred or not and you proceed accordingly, e.g.

Imports System.Net
Imports System.ComponentModel
 
Public Class Form1
 
    Private WithEvents downloader As New WebClient
 
    Private Sub downloadButton_Click(ByVal sender As Object, ByVal e As EventArgs) Handles downloadButton.Click
        If Not Me.downloader.IsBusy Then
            MessageBox.Show("A download is already in progress.",
                            "Download In Progress",
                            MessageBoxButtons.OK,
                            MessageBoxIcon.Error)
        Else
            Try
                Me.downloadButton.Enabled = False
 
                Dim remoteAddress As String = Me.remoteAddressTextBox.Text
                Dim localAddress As String = Me.localAddressTextBox.Text
                Dim userToken As String() = New String() {remoteAddress, localAddress}
 
                Me.downloader.DownloadFileAsync(New Uri(remoteAddress), localAddress, userToken)
            Catch ex As Exception
                Me.downloadButton.Enabled = True
 
                MessageBox.Show(ex.Message, "Download Failed", MessageBoxButtons.OK, MessageBoxIcon.Error)
            End Try
        End If
    End Sub
 
    Private Sub downloader_DownloadFileCompleted(ByVal sender As Object,
                                                 ByVal e As AsyncCompletedEventArgs) Handles downloader.DownloadFileCompleted
        Me.downloadButton.Enabled = True
 
        If e.Error Is Nothing Then
            Dim addresses As String() = DirectCast(e.UserState, String())
 
            MessageBox.Show(String.Format("The file '{0}' was successfully downloaded to '{1}'.",
                                          addresses(0),
                                          addresses(1)),
                            "Download Complete",
                            MessageBoxButtons.OK,
                            MessageBoxIcon.Error)
        Else
            MessageBox.Show(e.Error.Message, "Download Failed", MessageBoxButtons.OK, MessageBoxIcon.Error)
        End If
    End Sub
 
End Class

With regards to the NetworkAvailabilityChanged event, there may not be any reason to handle it as the DownloadFileCompleted event will still provide exception details, so you may end up double-reporting to the user. If you do still want to handle it, you can check the IsBusy property of your WebClient to determine whether a file is being downloaded.