I want to ping every IP on a subnet (255 IP's) and display in a table what
IP's are active.  I'm currently using a For loop but this takes forever
because it pauses to test each Ping.  So my guess is that I need to run 255
functions each in their own thread.  Would there be an easy way to do this,
maybe create the functions dynamically within a loop?  Any help with the
syntax would be great.  I haven't done much threaded programming.

Do you need this functionality as part of an existing app or are you trying to
write something that does this?  There are already apps out there that do this
very quickly, like nmap.

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Hi Ryan,

For such scenario, I recommend use several BackgroundWorker instances to do
the ping and update UI to report IP address's status.

First, we need to create a class to provide the next IP address that needs
to ping:

    Public Class IpProvider
        Private _index As Integer = 0
        Public ReadOnly Property GetNextIpAddress() As String
            Get
                SyncLock GetType(IpProvider)
                    If _index = 255 Then Return Nothing
                    _index = _index + 1
                    Return "192.168.1." + _index.ToString()
                End SyncLock
            End Get
        End Property
    End Class

Here I'm using a hardcoded subnet for demostrating purpose. You may need to
use some properties to set the specified subnet.

Add a ListBox to your Form which will display each IP address's status. Add
two buttons, the first one will be used to start the ping threads, the
second one will be used to cancel the ping threads. Here we will use 5
threads to do the ping, each thread will use the IpProvider to get the next
ip address to ping.

    Const THREAD_COUNT As Integer = 5
    Private bws(THREAD_COUNT - 1) As BackgroundWorker

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles Button1.Click
        Dim ipp As New IpProvider

        For i As Integer = 0 To THREAD_COUNT - 1
            Dim bw As New BackgroundWorker
            bw.WorkerReportsProgress = True
            bw.WorkerSupportsCancellation = True

            AddHandler bw.DoWork, AddressOf bw_DoWork
            AddHandler bw.ProgressChanged, AddressOf bw_ProgressChanged
            AddHandler bw.RunWorkerCompleted, AddressOf
bw_RunWorkerCompleted
            bw.RunWorkerAsync(ipp)

            bws(i) = bw
        Next
    End Sub

    Private Sub bw_DoWork(ByVal sender As Object, ByVal e As
DoWorkEventArgs)
        Dim bw As BackgroundWorker = CType(sender, BackgroundWorker)
        e.Result = PingIp(e.Argument, bw, e)
    End Sub

    Private Sub bw_ProgressChanged(ByVal sender As Object, ByVal e As
ProgressChangedEventArgs)
        ListBox1.Items.Add(e.UserState)
    End Sub

    Private Sub bw_RunWorkerCompleted(ByVal sender As Object, ByVal e As
RunWorkerCompletedEventArgs)
        If (e.Error IsNot Nothing) Then
            ListBox1.Items.Add(e.Error.Message)
        ElseIf e.Cancelled Then

        Else

        End If
    End Sub

    Private Function PingIp(ByVal ipp As IpProvider, ByVal bw As
BackgroundWorker, ByVal e As DoWorkEventArgs) As Boolean
        While True
            If bw.CancellationPending Then
                e.Cancel = True
                Return False
            Else
                Dim ipaddress = ipp.GetNextIpAddress()
                If ipaddress Is Nothing Then Exit While
                ' insert code to ping the address, here we're using Sleep()
to simulate a delay
                Thread.Sleep(1000)

                ' then we report the result, here we don't care the
progress percentage
                bw.ReportProgress(0, ipaddress + " is alive")
            End If
        End While
        Return True
    End Function

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles Button2.Click
        For i As Integer = 0 To THREAD_COUNT - 1
            bws(i).CancelAsync()
        Next
    End Sub

Hope this helps. Please feel free to post here if anything is unclear.

Sincerely,
Walter Wang (waw***@online.microsoft.com, remove 'online.')
Microsoft Online Community Support

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