int port;

while (!isAvailable)
{
  port = _rnd.Next(50000, 51000);

  isAvailable = !(from p in
                IPGlobalProperties.GetIPGlobalProperties().GetActiveUdpListeners()
                where p.Port == port
                select p).Any();
}

System.NotSupportedException: This platform is not supported
  at System.Net.NetworkInformation.IPGlobalProperties.GetIPGlobalProperties ()