Problem using Parallel.ForEachAsync


I have a problem using Parallel.ForEachAsync.

I have a list of 3500 url's, and associated data. They are stored in "links" which is a List(Of link).I wish to simply validate each url to confirm they exist. I would like to exploit parallelism and test them (say) 10 at a time. I have developed the following code, but I cannot get it to work.

At the statement "Async Sub(t)" I get a BC36670 error, and nothing I can do seems to clear it. I'm new to parallelism, and I can't understand what the error message is trying to tell me. Any insights?


                count = 0
                Dim options As New ParallelOptions()
                options.MaxDegreeOfParallelism = 10
                Await Parallel.ForEachAsync(links, options,
                       Async Sub(t)
                           count += 1
                           'AppendText(TextBox2, $"Processing index {t.index}{vbCrLf}")
                           If t.url = "" Then
                               t.status = "<font color='orange'>Missing</font>"
                               Missing += 1
                               'Dim r = Await TestURL(t.url)
                               Dim httpResult = Await httpclient.GetAsync(t.url)
                               Dim r = httpResult.IsSuccessStatusCode
                               t.OK = r
                               If r Then
                                   t.status = "<font color='green'>OK</font>"
                                   OK += 1
                                   t.status = "<font color='red'>Not found</font>"
                                   NotFound += 1
                               End If
                               SetText(TextBox1, $"Processed {count}/{links.Count}")
                           End If

                       End Sub)
            Catch ex As Exception
                MsgBox(ex.Message & vbCrLf & ex.StackTrace, vbCritical + vbOK, "Parallel web access failed")
            End Try

            ' Produce results
            logWriter.WriteLine("<table border=1>")
            For Each lnk In links
                logWriter.WriteLine($"<tr><td>{lnk.WWFFID}</td><td>{lnk.LongName}</td><td>{lnk.State}</td><td><a href='{lnk.url}'>{lnk.url}</a></td><td>{lnk.status}</td></tr>")
            logWriter.WriteLine($"<br>{links.Count} total links validated. {OK} OK, {Missing} Missing, {NotFound} Not Found")
            SetText(TextBox1, "Done")
        End Using
        Return True
    End Function
    Public Class link
        Property index As Integer
        Property url As String
        Property WWFFID As String
        Property LongName As String
        Property State As String
        Property status As String
        Property OK As Boolean
        Sub New(index As String, url As String, WWFFID As String, LongName As String, State As String)
            _index = index
            _url = url
            _WWFFID = WWFFID
            _LongName = LongName
            _State = State
            _status = status
            _OK = False
        End Sub
    End Class




1 Reply

Hi @vk3ohm,

Thanks for posting your issue here.

However this platform is used for how-to discussions and sharing best practices for building any app with .NET.Since your issue is a technical question, welcome to post it in Microsoft Q&A forum, the support team and communities on Microsoft Q&A will help you for any technical questions.
Besides, it will be appreciated if you can share it here once you post this technical question Microsoft Q&A.
Best Regards,
Lan Huang