Socket.Exceptionでハンドルされていない例外

private void button1_Click(object sender, System.EventArgs e)
{
	//DEBUG
	listBox1.Items.Add(Dns.GetHostName());
	IPHostEntry iph = Dns.Resolve(Dns.GetHostName());
	IPAddress[] ips = iph.AddressList;

	//DEBUG
	foreach (IPAddress ip in ips)
	{
		listBox1.Items.Add(ip.ToString());
	}

	IPEndPoint ipe = new IPEndPoint(ips[0],21);

	Socket sc =   new Socket(AddressFamily.InterNetwork,
                                  SocketType.Stream,ProtocolType.Tcp);
	try{
		sc.Connect(ipe);
		byte[] b = new byte[sc.Available];
		sc.Receive(b);
		listBox1.Items.Add(System.Text.Encoding.ASCII.GetString(b));
	}catch(Exception ex){
		SocketException se = null;
		if ((se = ex as SocketException) != null){
			MessageBox.Show(ex.Message +
                                            ": ErrCode = " + se.ErrorCode);
		}
	}finally{
		sc.Close();
	}
}