The code in this snippet (and attached project zip) demonstrates how to use a background worker thread to monitor a network address (name or IP). It shows how to start and stop the thread and how to update controls in the main thread using delegates. It should be simple to use this code as a template for creating background threads for other tasks.

A couple of quick notes:

You may wonder why I have tab characters padding out my comments. It becomes more obvious when you set the comments to display with black foreground and light grey (or silver) background. When you do this it becomes a little prettier to look at. I prefer shaded comments because it more easily distinguishes code from comments, and makes it more obvious when code is commented out.

<edit>It looks like trailing blanks/tabs are automatically removed</edit>

The attached file is named as "yyyy-mm-dd hh-mm". I don't use a CMS. When I get to a checkpoint I have a script that archives a folder. It prefixes the project folder name with the current date and time.

'  Name:                                                                        
'    BGPing                                                                     
'  Description:                                                                 
'    Sample code for starting and stopping a background thread. Also shows how  
'    to update foreground controls from background threads using delegates.     
'  Audit:                                                                       
'    2012-11-18  Reverend Jim - original code                                   

Public Class Form1

    'Background threads cannot write to controls in other threads. In order to  
    'update the status label you have to access it through a delegate. You'll   
    'see how this works in the UpdateStatus Sub.                                

    Private Delegate Sub dlgUpdateStatus(text As String)

    Private Sub Form1_FormClosing(sender As Object, e As System.Windows.Forms.FormClosingEventArgs) Handles Me.FormClosing

        'If the background thread is active, kill it. We already have the code  
        'in the button click handler so let's just use that.                    

        If btnStart.Text = "Stop" Then
        End If

    End Sub

    Private Sub btnStart_Click(sender As System.Object, e As System.EventArgs) Handles btnStart.Click

        'We'll start and stop the background worker using one button. We'll     
        'toggle the button text as we change states between running to stopped. 

        Dim btn As Button = sender

        If btn.Text = "Start" Then      'start the background thread
            btn.Text = "Stop"
            btn.Text = "Start"
            bgwPing.CancelAsync()       'tell the background thread to stop itself
        End If

    End Sub

    Private Sub UpdateStatus(text As String)

        'The InvokeRequired method returns True if the thread that calls it     
        'is running from a different thread. In that case we create a delegate  
        'to do the update of the control's text otherwise we can update the     
        'text directly.                                                         

        If lblStatus.InvokeRequired Then
            Dim dlg As New dlgUpdateStatus(AddressOf UpdateStatus)
            Me.Invoke(dlg, text)
            lblStatus.Text = TimeOfDay & " " & txtAddress.Text & " is " & text
        End If

    End Sub

    Private Sub bgwPing_DoWork(sender As System.Object, e As System.ComponentModel.DoWorkEventArgs) Handles bgwPing.DoWork


            'If the foreground thread requested a cancellation then set the     
            'appropriate flag and exit the loop.                                

            If bgwPing.CancellationPending Then
                e.Cancel = True
                Exit Do
            End If

            'do the ping and update the status label

            If My.Computer.Network.Ping(txtAddress.Text) Then
                UpdateStatus("Not Responding")
            End If

            'suspend for five seconds before the next ping



    End Sub

End Class
About the Author
Member Avatar Reverend Jim Hi, I'm Jim, one of DaniWeb's moderators.

I completed my Computer Science degree at the University of Manitoba in 1976. I did two and a half years of programming in medical research followed by twenty-nine years at Manitoba Hydro (electric utility). Most of that was spent on doing development and maintenance on an AGC/SCADA (real-time programming/process control) system. The last ten years of that was spent doing application and infrastructure support and development. I have programmed in FORTRAN (mostly), APL, PL/1, COBOL, Lisp, SNOBOL, ALGOL, Assembler (several flavours), C, C++, Paradox, VB, vbScript and more recently, Python. I am married with two grown children of whom I am very proud, and a most beautiful wife. I am currently retired (and loving it).

Good code to understand the concept of delegates for those who never used in real time.

Thanks. It seems a lot of examples of code in books are like this.

john.knapp commented: LOL +2