This C# class combined with the Arduino sketch allows you to plug an Arduino into a PC's USB port and then have its com port detected by the C# application. Once the com port is known the C# application can then send/receive messages from the Arduino.

I used an Arduino Uno, C# 4.0, Windows 7.

This code does not work on Leonardo's because, apparently, a Leonardo requires setting the DtrEnable property of the SerialPort object to true. I don't have a Leonardo so can't test this but I believe it to be true based on the forum posts.

The concept is quite simple. The C# application polls all the com ports and sends a query using a simple protocol (explained below). Once the Arduino receives the request it replies. The C# application that notes the com port that caused the Arduino to reply and saves the name of the com port. The C# application can then use this com port in subsequent communications.

Comms Protocol

This uses a 4 byte message Byte 0 is the start of command marker. This is always decimal 16 converted to byte (Convert.ToByte(16);)

Byte 1 is the type of command:
127 = Send data to pins
128 = Identify

Byte 2 is the pin to receive data
Byte 3 is the valus for the pin

So send 100 to pin 3 is:
16,127,3,100

And ask the Arduino to return the identify message is:
16,128,0,0

I note the code uses an extra Byte 4. This was an end of message marker but is not used

You can easily add to the protocol. In one projects I use command 129 to request data back from the Arduino.

If you have questions send me a personal message via the forum Richard210363

Sketch

/*

 * Serial Port Monitor
 *
 * 
 */

//Setup Output int ledPin_3 = 3;

//Setup message bytes

byte inputByte_0;

byte inputByte_1;

byte inputByte_2;

byte inputByte_3;

byte inputByte_4;

//Setup

void setup() {

  pinMode(ledPin_3, OUTPUT);
  Serial.begin(9600);
  digitalWrite(ledPin_3, HIGH);//
  delay(250);//
  digitalWrite(ledPin_3, LOW);//
  delay(250);//

}

//Main Loop

void loop() {

  //Read Buffer
  if (Serial.available() == 5) 
  {
    //Read buffer
    inputByte_0 = Serial.read();
    delay(100);    
    inputByte_1 = Serial.read();
    delay(100);      
    inputByte_2 = Serial.read();
    delay(100);      
    inputByte_3 = Serial.read();
    delay(100);
    inputByte_4 = Serial.read();   
  }
  //Check for start of Message
  if(inputByte_0 == 16)
  {       
       //Detect Command type
       switch (inputByte_1) 
       {
          case 127:
             //Set PIN and value
             switch (inputByte_2)
            {
              case 4:
                if(inputByte_3 == 255)
                {
                  digitalWrite(ledPin_3, HIGH); 
                  break;
                }
                else
                {
                  digitalWrite(ledPin_3, LOW); 
                  break;
                }
              break;
            } 
            break;
          case 128:
            //Say hello
            Serial.print("HELLO FROM ARDUINO");
            break;
        } 
        //Clear Message bytes
        inputByte_0 = 0;
        inputByte_1 = 0;
        inputByte_2 = 0;
        inputByte_3 = 0;
        inputByte_4 = 0;
        //Let the PC know we are ready for more data
        Serial.print("-READY TO RECEIVE");
  }

}

C#

using System;

using System.Threading;

using System.IO.Ports;

using System.IO;

public class ArduinoControllerMain {

	SerialPort currentPort;
	bool portFound;

	private void SetComPort()
	{
	    try
	    {
		string[] ports = SerialPort.GetPortNames();
		foreach (string port in ports)
		{
		    currentPort = new SerialPort(port, 9600);
		    if (DetectArduino())
		    {
			portFound = true;
			break;
		    }
		    else
		    {
			portFound = false";
		    }
		}
	    }
	    catch (Exception e)
	    {
	    }
	}

	private bool DetectArduino()
	{
	    try
	    {
		//The below setting are for the Hello handshake
		byte[] buffer = new byte[5];
		buffer[0] = Convert.ToByte(16);
		buffer[1] = Convert.ToByte(128);
		buffer[2] = Convert.ToByte(0);
		buffer[3] = Convert.ToByte(0);
		buffer[4] = Convert.ToByte(4);

		int intReturnASCII = 0;
		char charReturnValue = (Char)intReturnASCII;

		currentPort.Open();
		currentPort.Write(buffer, 0, 5);
		Thread.Sleep(1000);

		int count = currentPort.BytesToRead;
		string returnMessage = "";
		while (count > 0)
		{
		    intReturnASCII = currentPort.ReadByte();
		    returnMessage = returnMessage + Convert.ToChar(intReturnASCII);
		    count--;
		}
		ComPort.name = returnMessage;

		currentPort.Close();

		if (returnMessage.Contains("HELLO FROM ARDUINO"))
		{
		    return true;
		}
		else
		{
		    return false;
		}
	    }
	    catch (Exception e)
	    {
		return false;
	    }
      }

}