This article was previously published under Q194938
This article has been archived. It is offered "as is" and will no longer be updated.
This article describes how to monitor whether a TCP/IP port is in use.
There are times when it is necessary to know or monitor whether a port isin use. The following sample demonstrates how this is done.
Create a new Standard EXE project. Form1 is created by default.
Add 1 Timer, 2 Labels, 1 Listbox, 1 TextBox and 2 CommandButton controls to Form1. Position Label1 above Text1 and Label2 above List1.
Set the Sorted property of List1 to True.
Add the following code to the General Declarations section of Form1:
Option Explicit Const PortsChecked = 200 Private Sub Command1_Click() Timer1.Enabled = True Timer1.Interval = 1000 End Sub Private Sub Command2_Click() Timer1.Interval = 0 Timer1.Enabled = False End Sub Private Sub Timer1_Timer() Dim X As Integer List1.Clear For X = 1 To PortsChecked DoEvents Text1.Text = X WinSock1.LocalPort = X On Error Resume Next WinSock1.Listen ' If we get an error, the port is busy. If Err.Number = 10048 Then List1.AddItem X ' Log Active port # to list box. Err.Number = 0 End If WinSock1.Close Next X End Sub Private Sub Form_Load() Label1.Caption = "Checking Port #" Label2.Caption = "Ports In Use" Command1.Caption = "Start" Command2.Caption = "End" Text1.Locked = True End Sub
Run the program. If, for example, you have an active DCOM connection, you should see 135 displayed in the ListBox.
To change the number of ports checked, modify the constant PortsChecked. To change the amount of time between samples modify the constant TimerInt.
For additional information, please see the following article in theMicrosoft Knowledge Base:
173619 PRB: Winsock Control Generates Error 10048 - Address in Use
Also search Visual Basic Help for "Using the Winsock Control."