DownloadFileAsync not so Async

[Syzis]

Hello,

I'm having some trouble while downloading files async.

I have the following code:

    Private Sub DownLoadFileAsync(address As String, path As String)

        Dim client As Net.WebClient = New Net.WebClient()

        AddHandler client.DownloadDataCompleted, Sub(sender As Object, e As System.Net.DownloadDataCompletedEventArgs)
                                                     RaiseEvent OnDownloadCompleted(e.Result)
                                                 End Sub

        AddHandler client.DownloadProgressChanged, Sub(sender As Object, e As System.Net.DownloadProgressChangedEventArgs)
                                                       RaiseEvent OnDownloadProgress(e.ProgressPercentage)
                                                       Application.DoEvents()
                                                   End Sub

        Dim uri As Uri = New Uri(address)
        client.DownloadFileAsync(uri, path)
    End Sub

Then, I have the following code:

        MetroSetProgressBar1.Value = 0

        AddHandler Me.OnDownloadProgress, Sub(percet As Integer)
                                              MetroSetProgressBar1.Value = percet
                                          End Sub


        AddHandler Me.OnDownloadCompleted, Sub(data() As Byte)
                                               MetroSetProgressBar1.Value = 100
                                           End Sub





        Dim selected As String = MetroSetListBox1.Items(MetroSetListBox1.SelectedIndex()).ToString()
        Dim splited() As String = selected.Split(" - ")

        Dim card As String = MetroSetListBox2.Items(MetroSetListBox2.SelectedIndex()).ToString()
        Dim cardsplit() As String = card.Split(" - ")


        Dim uriString As String = "API URL"

        Dim Request As HttpWebRequest = WebRequest.Create(New Uri(uriString))
        Dim JSON_Response As String = New StreamReader(Request.GetResponse().GetResponseStream()).ReadToEnd()

        Dim JSON_Obj1 As Object = New JavaScriptSerializer().DeserializeObject(JSON_Response)
        Dim Test1 As String = JSON_Obj1("image")("png").ToString()

        Dim savePathimg As String = MetroSetTextBox1.Text & splited(0) & "\" & cardsplit(0) & ".png"
        Dim savePath As String = MetroSetTextBox1.Text & splited(0)

        If (Not System.IO.Directory.Exists(savePath)) Then
            If My.Settings.create_folder = 1 Then
                System.IO.Directory.CreateDirectory(savePath)
                Dim download As New WebClient()
                AddHandler download.DownloadProgressChanged, AddressOf Download_ProgressChanged
                'download.DownloadFileAsync(New Uri(Test1), savePathimg)
                DownLoadFileAsync(Test1, savePathimg)
                If My.Settings.open_location = 1 Then
                    Process.Start("explorer.exe", savePath)
                End If
            Else
                MetroSetMessageBox.Show(Me, "Folder " & savePath & " does not exists. Please create it or go to options.", "Warning", MessageBoxButtons.OK, MessageBoxIcon.Error)
            End If
        Else
            Dim download As New WebClient()
            AddHandler download.DownloadProgressChanged, AddressOf Download_ProgressChanged
            download.DownloadFileAsync(New Uri(Test1), savePathimg)
            If My.Settings.open_location = 1 Then
                Process.Start("explorer.exe", savePath)
            End If
        End If

I'm consuming an API that downloads some files. It could 5, it could be 300, one at a time. My UI frozes while the downloads are made, so it's not making it async.
So, what I'm doing wrong?
Also, how can I make this to wait X seconds between each download without frozing my UI?

Thanks

[NeedSomeAnswers]

Well your not awaiting your response, just calling the async method doesn't make it async unless you await the response

Take a look at this https://docs.microsoft.com/en-us/dot...oncepts/async/

[Syzis]

I read the link you provided but got stuck. Can't transform my private sub into a task.
I don't have so much knowledge in programming to make it work.

[Niya]

'
    Async Private Function DownLoadFileAsync(address As String, path As String) as Task
 
        Dim client As Net.WebClient = New Net.WebClient()
 
        AddHandler client.DownloadDataCompleted, Sub(sender As Object, e As System.Net.DownloadDataCompletedEventArgs)
                                                     RaiseEvent OnDownloadCompleted(e.Result)
                                                 End Sub
 
        AddHandler client.DownloadProgressChanged, Sub(sender As Object, e As System.Net.DownloadProgressChangedEventArgs)
                                                       RaiseEvent OnDownloadProgress(e.ProgressPercentage)
                                                       Application.DoEvents()
                                                   End Sub
 
        Dim uri As Uri = New Uri(address)
        Await client.DownloadFileAsync(uri, path)
    End Sub

Note: I altered your code in plain text outside the IDE so no auto correct. I'm not sure if it's Async Private Function or Private Async Function but you can correct to suit. I still do most of my asynchronous stuff the old fashion way so I don't use this as much yet.

Also, when you're using that make sure that whatever is calling that method is also using Await on the method.

[peterst]

Just to mention that System.Net.WebClient.DownloadFileAsync is not Async so it cannot be awaited:

public void DownloadFileAsync(Uri address, string fileName) =>
     DownloadFileAsync(address, fileName, null);
 
public void DownloadFileAsync(Uri address, string fileName, object? userToken)
{
...

There is another function DownloadFileTaskAsync which returns Task and can be awaited.

[Inferrd]

I'd be very surprised if call(s) to DownLoadFileAsync caused the UI to freeze. I think the problem is much more likely to be found in code you haven't shown, or in the way you are using the code that you have shown here.

You say you might download up to 300 images. So are you looping the second block of code? I ask because the lines:

Dim Request As HttpWebRequest = WebRequest.Create(New Uri(uriString))
Dim JSON_Response As String = New StreamReader(Request.GetResponse().GetResponseStream()).ReadToEnd()

are not Async, and will freeze the UI while the web request is negotiated and the JSON is downloaded from the server. Do that once and you'd hardly notice. Doing it 300 times in a loop however, would be a big problem.

While running you code from the IDE, try pressing the pause button (or Debug menu/ Break All) at the point that the UI freezes. The line of code that is blocking execution or is part of a long running loop should be highlighted. Doesn't always work, but is worth a try.