Arduino's don't seem to trigger the callback onDataReceived.

SerialCallbackTest.cs

using System;
using System.IO.Ports;
using System.Threading;
using System.ComponentModel;

class Program
{
	public static void Main()
	{
		SerialPort mySerialPort = new SerialPort("/dev/tty.usbmodem621", 9600, Parity.None, 8, StopBits.One);
//		mySerialPort.BaudRate = 9600;
//		mySerialPort.Parity = Parity.None;
//		mySerialPort.StopBits = StopBits.One;
//		mySerialPort.DataBits = 8;
//		mySerialPort.WriteBufferSize = 1;
//		mySerialPort.ReadBufferSize = 1;
//		mySerialPort.Handshake = Handshake.None;
//		mySerialPort.RtsEnable = true;
//		mySerialPort.DtrEnable = false;
		mySerialPort.DataReceived += new SerialDataReceivedEventHandler(DataReceivedHandler);
//		mySerialPort.Disposed += SerialDataReceivedEventHandler(DataReceivedHandler);
		mySerialPort.Open();
		Console.WriteLine("Press any key to continue...");
		Console.WriteLine();
		mySerialPort.WriteLine ("a");
		System.Threading.Thread.Sleep(500);
		mySerialPort.WriteLine ("a");
		Console.ReadKey();
		mySerialPort.Close();
	}

	private static void DataReceivedHandler(
		object sender,
		SerialDataReceivedEventArgs e)
	{
		SerialPort sp = (SerialPort)sender;
		string indata = sp.ReadExisting();
		Console.WriteLine("Data Received:");
		Console.Write(indata);
	}
}