Today I wanted a way to see which of my cameras were online at a certain point in time. Pinging them is a great way of doing that, it at least says they are there and the first two layers of the OSI model are working (if I remember correctly).
Doing a ping to another computer in VB.Net is easy.
Dim _ping As New Ping
Dim _pingreply = _ping.Send(IpAddress, 2000)
If _pingreply.Status = IPStatus.Success Then
Like this.```
The first parameter of Ping.Send is the IpAddress as string and the second is the timeout in this case 2 seconds.
I want to ping 5 cameras at a time to worst case this will take 10 seconds. 10 seconds is a long time to wait for a user who’s attention span is 3 seconds. So we need a way to do this asynchronously.
So I create a grid with 2 columns and 5 rows. First column has the Ipaddresses and the second will show the result.
Here is how.
```vbnet
Private Sub Form1_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
fGrid.Columns.Add("Ip", "Ip")
fGrid.Columns.Add("Ping", "Ping")
fGrid.Columns(0).Width = 100
fGrid.Columns(1).AutoSizeMode = DataGridViewAutoSizeColumnMode.Fill
For i As Integer = 0 To 4
Dim myRowIndex = fGrid.Rows.Add()
fGrid.Rows(myRowIndex).Cells(0).Value = "10.216.110." & (11 + myRowIndex).ToString
Next
End Sub```
Looks like this.
<div class="image_block">
<a href="/wp-content/uploads/users/chrissie1/ping/ping1.png?mtime=1302788168"><img alt="" src="/wp-content/uploads/users/chrissie1/ping/ping1.png?mtime=1302788168" width="425" height="410" /></a>
</div>
So now that we have that we can do the pings in another thread. So I made a button to spawn this thread and begin the process.
```vbnet
Private fThread = New Thread(New ThreadStart(AddressOf ThreadProc))
fThread.IsBackground = True
fThread.Start()
```
and this will use the following things.
```vbnet
Private Sub ThreadProc()
Parallel.For(0, 5, Sub(b)
CheckOnline(b)
End Sub)
End Sub
Private Sub CheckOnline(ByVal rowindex As Integer)
Dim _ping As New Ping
Try
Dim _pingreply = _ping.Send("10.216.110." & (11 + rowindex).ToString, 2000)
If _pingreply.Status = IPStatus.Success Then
SyncLock success
success.Add(rowindex)
End SyncLock
End If
Catch ex As Exception
End Try
End Sub```
Yep I used a parallel foreach that will spawn 5 more threads to do the pings in parallel for me.
But I also needed a way to tell my users I was doing something and a way to tell them it was all finished and the result.
I did that with a second thread that check the result in a list and sees if they are done, making sure I synchronize the lists add method because that is not threadsafe.
```vbnet
Imports System.Threading
Imports System.Threading.Tasks
Imports System.Net.NetworkInformation
Partial Public Class Form1
Private fThread As Thread
Private fThread2 As Thread
Public Delegate Sub AddRowDelegate(ByVal column As Integer)
Delegate Sub CheckOnlineDelegate(ByVal rowindex As Integer)
Delegate Sub SetOnlineDelegate(ByVal rowindex As Integer)
Private Sub Form1_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
fGrid.Columns.Add("Ip", "Ip")
fGrid.Columns.Add("Ping", "Ping")
fGrid.Columns(0).Width = 100
fGrid.Columns(1).AutoSizeMode = DataGridViewAutoSizeColumnMode.Fill
For i As Integer = 0 To 4
Dim myRowIndex = fGrid.Rows.Add()
fGrid.Rows(myRowIndex).Cells(0).Value = "10.216.110." & (11 + myRowIndex).ToString
Next
End Sub
Private Sub ThreadProc()
Parallel.For(0, 5, Sub(b)
Do While Not done.Contains(b)
fGrid.Invoke(New AddRowDelegate(AddressOf AddRow), New Object() {b})
Thread.Sleep(300)
Loop
fGrid.Invoke(New SetOnlineDelegate(AddressOf SetOnline), New Object() {b})
End Sub)
End Sub
Private Sub ThreadProc2()
Parallel.For(0, 5, Sub(b)
CheckOnline(b)
End Sub)
End Sub
Private Sub AddRow(ByVal rowindex As Integer)
If fGrid.Rows(rowindex).Cells(1).Value Is Nothing OrElse fGrid.Rows(rowindex).Cells(1).Value.ToString.Contains(".....") OrElse Not fGrid.Rows(rowindex).Cells(1).Value.ToString.Contains("Pinging") Then
fGrid.Rows(rowindex).Cells(1).Value = "Pinging 10.216.110." & (11 + rowindex).ToString & " "
Else
fGrid.Rows(rowindex).Cells(1).Value = fGrid.Rows(rowindex).Cells(1).Value.ToString & "."
End If
fGrid.Rows(rowindex).Cells(1).Style.BackColor = Drawing.Color.White
End Sub
Private success As New List(Of Integer)
Private done As New List(Of Integer)
Private Sub CheckOnline(ByVal rowindex As Integer)
Dim _ping As New Ping
Try
Dim _pingreply = _ping.Send("10.216.110." & (11 + rowindex).ToString, 2000)
If _pingreply.Status = IPStatus.Success Then
SyncLock success
success.Add(rowindex)
End SyncLock
End If
Catch ex As Exception
End Try
SyncLock done
done.Add(rowindex)
End SyncLock
End Sub
Private Sub SetOnline(ByVal rowindex As Integer)
If Not success.Contains(rowindex) Then
fGrid.Rows(rowindex).Cells(1).Value = "Offline"
fGrid.Rows(rowindex).Cells(1).Style.BackColor = Drawing.Color.Red
Else
fGrid.Rows(rowindex).Cells(1).Value = "Online"
fGrid.Rows(rowindex).Cells(1).Style.BackColor = Drawing.Color.Green
End If
End Sub
Private Sub PingToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles PingToolStripMenuItem.Click
done = New List(Of Integer)
success = New List(Of Integer)
fThread = New Thread(New ThreadStart(AddressOf ThreadProc))
fThread.IsBackground = True
fThread.Start()
fThread2 = New Thread(New ThreadStart(AddressOf ThreadProc2))
fThread2.IsBackground = True
fThread2.Start()
End Sub
End Class
As you can see I invoke the SetOnline and AddRow methods via the grid so that it can be handled in a thread safe way. I also use delegates for that.
While running it you will see that if one is online it will turn green immediately or very fast.
Look also that the Parallel For second parameter is Exclusive To so it does not translate into For i as integer = 0 to 5 but it translates into For i as integer = 0 to 4.
And that you have to wait for about 2 (timeout we used) seconds for all the others to turn red.
Writing thread safe code is tricky but I think I covered all the bases to make this as thread safe as possible without getting race conditions.
Any questions?
Chris is awesome.
