Threadpool and Threading in general

[stateofidleness]

whipped up this code real quick which uses a ThreadPool on Form Load to iterate through my subnet and tell me if there's a device communicating at each IP address.

Works great.

some questions:
Why am I able to update BOTH my label at the top AND the listbox, but I'm only using an .Invoke on the Listbox?

How can I know how many threads are currently being used while that is executing?

What does the SyncLock do?

I'm aware some people don't like the use of Application.DoEvents(). Is it ok to be using it in this instance or is there a better practice?

For those wanting to try it out, drag a Label and a Listbox to a new form. Label is called "currentIPTextbox", Listbox is default name.

Imports System.Threading
 
Public Class Form1
    Private Elem As String
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        ListBox1.Items.Clear()
        ThreadPool.QueueUserWorkItem(AddressOf Initialize)
    End Sub
 
    Private Sub Add()
        Dim pingresult As String
        pingresult = My.Computer.Network.Ping("192.168.1." & Elem, 200)
        ListBox1.Items.Add("192.168.1." & Elem & " : " & pingresult)
        currentIPTextbox.Text = "192.168.1." & Elem
        Application.DoEvents()
    End Sub
 
    Private Sub Initialize(ByVal State As Object)
        Dim I As Long
        SyncLock ListBox1.GetType
            For I = 1 To 255
                Elem = I
                ListBox1.Invoke(CType(AddressOf Add, MethodInvoker))
            Next
        End SyncLock
    End Sub
End Class