How to check if a TCP port is blocked using C# or VB.NET

In order to check if a TCP port is blocked, use the System.Net and System.Net.Sockets classes as shown below:

C#

protected void Button1_Click(object sender, EventArgs e)

{

string host = "localhost";

int port = 6900;

IPAddress addr = (IPAddress)Dns.GetHostAddresses(host)[0];

try

{

TcpListener tcpList = new TcpListener(addr, port);

tcpList.Start();

}

catch (SocketException sx)

{

// Catch exception here if port is blocked

}

}

VB.NET

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs)

Dim host As String = "localhost"

Dim port As Integer = 6900

Dim addr As IPAddress = CType(Dns.GetHostAddresses(host)(0), IPAddress)

Try

Dim tcpList As New TcpListener(addr, port)

tcpList.Start()

Catch sx As SocketException

' Catch exception here if port is blocked

End Try

End Sub