Gets the next available port number

TcpPort.cs

public static class TcpPort
{
  public static int NextFreePort()
  {
    var l = new TcpListener(IPAddress.Loopback, 0);
    try
    {
      l.Start();
      
      return ((IPEndPoint)l.LocalEndpoint).Port;
    }
    finally
    {
      l.Stop();
    }
  }
}

TcpPort.fs

open System.Net
open System.Net.Sockets

let getFreePort = fun _ ->
  let l = new TcpListener(IPAddress.Loopback, 0)
  let port =
    try
      l.Start()
      let endpoint = l.LocalEndpoint :?> IPEndPoint
      endpoint.Port
    finally
      l.Stop()
  port