How can I cancel a thread?

Question

I do something in a thread. But sometimes I don't want to wait till all pings are finished.

Can you show me please the code?

Private Sub Start_Button_Click(sender As Object, e As EventArgs) Handles Start_Button.Click
    DoSomething()
End Sub
Private Sub Cancel_Button_Click(sender As Object, e As EventArgs) Handles Cancel_Button.Click
    THRD.Cancel '<-- Thread cancel!??!???
End Sub
Sub DoSomething()
    Dim THRD As New Thread(Sub()
                               Dim IPArea As String = "192.168.1."
                               Dim LastIP As Integer
                               For LastIP = 0 To 255
                                   Dim TestIP As String = IPArea & CStr(LastIP)
                                   If My.Computer.Network.Ping(TestIP, 10) Then
                                       ListBox1.Items.Add(TestIP)
                                   End If
                               Next
                           End Sub)
    THRD.IsBackground = True
    THRD.Start()
End Sub

This answer is a good read: https://stackoverflow.com/questions/3632149/question-about-terminating-a-thread-cleanly-in-net For starters you will need to change the scope of your `THRD` variable in order to call it from the button's click event. — Pete -S-, Sep 05 '20 at 19:40
Exactly this what i don´t know how to do it and I can´t do anything with c#-code — vbnet_newer, Sep 05 '20 at 20:50
Are you getting any type of other errors like Cross-thread operation not valid: Control 'ListBox1' accessed from a thread other than the thread it was created on.'? — Pete -S-, Sep 05 '20 at 21:10
Only one error from the cancel_button because the code of THRD.Cancel is wrong. — vbnet_newer, Sep 05 '20 at 21:17
Have you run this code without the `THRD.Cancel '<-- Thread cancel!??!???` in place? — Pete -S-, Sep 05 '20 at 21:27
[Ping.SendAsync](https://learn.microsoft.com/en-us/dotnet/api/system.net.networkinformation.ping.sendasync) + [Ping.SendAsyncCancel](https://learn.microsoft.com/en-us/dotnet/api/system.net.networkinformation.ping.sendasynccancel). Ditch the thread. — Jimi, Sep 05 '20 at 22:27

score 1 · Accepted Answer · answered Sep 05 '20 at 21:36

Here is my working solution and this solution is only to show how to move the THRD as a form level variable to allow stopping it when clicking the cancel button. I added some validations to prevent exceptions.

Public Class Form1
    Private THRD As Threading.Thread

    Private Sub Start_Button_Click(sender As Object, e As EventArgs) Handles Start_Button.Click
        DoSomething()
    End Sub
    Private Sub Cancel_Button_Click(sender As Object, e As EventArgs) Handles Cancel_Button.Click

        If THRD IsNot Nothing AndAlso THRD.IsAlive Then
            THRD.Abort()  '<-- Thread cancel!??!???
            THRD = Nothing
            AddToList("Stopped.")
        Else
            AddToList("Thread not running.")
        End If
    End Sub

    Sub DoSomething()
        If THRD IsNot Nothing AndAlso THRD.IsAlive Then
            AddToList("Still working...")
            Exit Sub
        End If

        THRD = New Threading.Thread(Sub()
                                        Dim IPArea As String = "192.168.1."
                                        Dim LastIP As Integer
                                        For LastIP = 0 To 255
                                            Dim TestIP As String = IPArea & CStr(LastIP)
                                            If My.Computer.Network.Ping(TestIP, 10) Then
                                                AddToList(TestIP)
                                            End If
                                        Next
                                        AddToList("Done")
                                    End Sub)
        THRD.IsBackground = True
        THRD.Start()
    End Sub

    ''' <summary>
    ''' Thead-safe add value to list.
    ''' </summary>
    ''' <param name="value">The value.</param>
    Private Sub AddToList(value As String)
        If ListBox1.InvokeRequired Then
            ListBox1.Invoke(Sub() ListBox1.Items.Add(value))
        Else
            ListBox1.Items.Add(value)
        End If
    End Sub
End Class

One issue you have to address is if you cancel the thread, you may get this error: ThreadAbortException: Thread was being aborted. It may be better to use a delegate and put in a try/catch. — Pete -S-, Sep 05 '20 at 21:46

How can I cancel a thread?

1 Answers1

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