Não há muito o que dizer aqui, a não ser que isso não funcione, e não tenho ideia do porquê.

A saída serial no Arduino não é nada. A saída no código C # desce para aguardar uma resposta e, em seguida, nada.

O LED da placa Bluetooth no Arduino fica verde quando eu inicio o programa C # para que haja uma conexão sendo feita. Apenas mais nada.

Código Arduino

#include <SoftwareSerial.h> //Software Serial Port
#define RxD 8 // This is the pin that the Bluetooth (BT_TX) will transmit to the Arduino (RxD)
#define TxD 7 // This is the pin that the Bluetooth (BT_RX) will receive from the Arduino (TxD)

SoftwareSerial blueToothSerial(RxD,TxD);
boolean light = false;
void setup(){
  Serial.begin(9600); // Allow Serial communication via USB cable to computer (if required)
  pinMode(RxD, INPUT); // Setup the Arduino to receive INPUT from the bluetooth shield on Digital Pin 6
  pinMode(TxD, OUTPUT); // Setup the Arduino to send data (OUTPUT) to the bluetooth shield on Digital Pin 7
  pinMode(13,OUTPUT); // Use onboard LED if required.
}

void loop(){
   delay(1000);
   if(light){
      light = false;
      digitalWrite(13,LOW);
   }
   else{
     light = true;
     digitalWrite(13,HIGH);
   }
   //Serial.println(blueToothSerial.available());
   blueToothSerial.write("Im alive");
   if(blueToothSerial.read()>0){
     Serial.write(blueToothSerial.read());
   }
}

Código C # Básico

static void Main(string[] args)
        {
        byte[] Command = Encoding.ASCII.GetBytes("It works");//{0x00,0x01,0x88};

        SerialPort BlueToothConnection = new SerialPort();
        BlueToothConnection.BaudRate = (9600);

        BlueToothConnection.PortName = "COM4";
        BlueToothConnection.Open();
        if (BlueToothConnection.IsOpen)
        {

            BlueToothConnection.ErrorReceived += new SerialErrorReceivedEventHandler(BlueToothConnection_ErrorReceived);

            // Declare a 2 bytes vector to store the message length header
            Byte[] MessageLength = { 0x00, 0x00 };

            //set the LSB to the length of the message
            MessageLength[0] = (byte)Command.Length;


            //send the 2 bytes header
            BlueToothConnection.Write(MessageLength, 0, MessageLength.Length);

            // send the message itself

            System.Threading.Thread.Sleep(2000);
            BlueToothConnection.Write(Command, 0, Command.Length);

            Messages(5, ""); //This is the last thing that prints before it just waits for response.

            int length = BlueToothConnection.ReadByte();
            Messages(6, "");
            // retrieve the reply data
            for (int i = 0; i < length; i++)
            {
                Messages(7, (BlueToothConnection.ReadByte().ToString()));
            }
        }
    }