Question [WebClient.DownloadStringAsync] Force to wait in loop?

[littlebigman]

Hello

I noticed something strange when using WebClient.DownloadStringAsync in a loop to download several web pages: In each iteration, I add the URL in a ListBox, proceed to call DownloadStringAsync(), wait for a shared string to be filled with the web page, use a regex to extract its Title, update a RichTextBox with this, and then loop back to the next URL to fetch.

The odd thing, is that the ListBox is already showing the next URL although the program hasn't yet updated the RichTextBox :-/ How can I tell the loop to wait until the RTB is refreshed?

Here's the code:

Imports System.Net
Imports System.IO
Imports System.Text.RegularExpressions

Public Class Form1
    Shared page As String

    Public Shared Sub AlertStringDownloaded(ByVal sender As Object, ByVal e As DownloadStringCompletedEventArgs)
        If e.Cancelled = False AndAlso e.Error Is Nothing Then
            page = CStr(e.Result)
        End If
    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim title As Regex = New Regex("<title>(.+?)</title>")
        Dim m As Match

        Dim webClient As New WebClient

        AddHandler webClient.DownloadStringCompleted, AddressOf AlertStringDownloaded

        Dim URLArray As New ArrayList
        URLArray.Add("http://www.google.com")
        URLArray.Add("http://www.yahoo.com")

        [COLOR="Blue"]'Why delay between update of ListBox and RichTextBox?[/COLOR]
        Dim URL As String
        For Each URL In URLArray
            ListBox1.Items.Add("Downloading " & URL)
            ListBox1.Refresh()

            page = Nothing
            webClient.DownloadStringAsync(New Uri(URL))
            [COLOR="blue"]'Better way to wait until page has been downloaded?[/COLOR]
            While page Is Nothing
                Application.DoEvents()
            End While

            'Use regex to extract bits of infos, and save them into SQLite
            m = title.Match(page)
            If m.Success Then
                RichTextBox1.AppendText(m.Groups(1).Value & vbCrLf)
                RichTextBox1.Update()
            End If

            [COLOR="blue"]'At this point, why is ListBox already updated with next URL?[/COLOR]
        Next
        Button1.Enabled = True

    End Sub
End Class

Thank you for any help.

 

[JohnH]

That code is basically defying the purpose of multithreading. Why call an async method just to wait to it completes? I replied in your other thread about handling the asynchronous download completed event, which is where you should process the downloaded string (and update richetextbox). Don't block the UI thread like you're doing in code above.

 

[littlebigman]

Mmm, yes, I'm not doing it right. I just wanted to avoid freezing the UI while keeping things as simple as possible since I don't know enough about VB.Net yet to go for a full-fledged threaded app. Thanks a lot for your help.