Arduino only works as expected through serial monitor

I've made a program to communicate to my Arduino in C#.
A very simple version of this worked as expected using these commands:
port.Write("1"); //(0, 1, 2, 3 etc..)

Now i've expanded on this by being able to send multiple characters to my arduino to construct words using an Array, but for some reason this only works when using the serial monitor and not with my C# program.
Here's the code: (Simply type LED to start controlling digital output 13, then use 0/1 to turn the LED off or on.)

char incByte;
char bufferArray[5];
String LEDfunction;
int index;
int LEDstate;
int blinkRate;
bool blinkState = false;
String aString;
String menuChoice = "X";

void setup() 
  {
    pinMode(LED_BUILTIN, OUTPUT);
    pinMode(13, OUTPUT);
    Serial.begin(9600);  
    Serial.print("Setting up\n\n");
  }

void byteConstructor(byte b)                    //Here multiple characters are turned into a string
{
  Serial.print("Building Array string");
  delay(100);
    switch (b)
    {
      case '\n':                                
      bufferArray[index] = 0;        
      aString = bufferArray;                    //Array to string       
      index = 0;
      break;
       default:
      if(index < 4)                             
        bufferArray[index++] = b;
      break;
    } 
}  
  
void loop() 
  {    
    if (Serial.available()>0) {
      byteConstructor(Serial.read());
      if(aString == "LED") menuChoice = "LED";
      if(menuChoice == "LED")
      {
        LEDfunction = aString;
        if(LEDfunction == "0") LEDstate = 0;
        if(LEDfunction == "1") LEDstate = 1;
        if(LEDfunction == "2") blinkState = false;
        if(LEDfunction == "3") blinkState = true;
        if(LEDfunction == "EXIT") menuChoice = "X";                
      }
    }

    if(blinkState == true)
    {
      Serial.print("Blinking\n");
      delay(500);
      digitalWrite(13, 0);
      delay(500);
    }
    if(blinkState == false)
    {
      delay(500);
    }
  
    digitalWrite(13, LEDstate);
    Serial.print("Latest input: ");
    Serial.println(aString); 
    Serial.print("end of the loop\n");    
}

Again this works as expected through the Arduino serial monitor but not when using my C# program, why is that?

By not working in C# you mean you don't get any data in your C# program, right?

On the arduino side you need to close the serial monitor and probably the arduino IDE as well. That's because the serial monitor will keep the port busy.

The Serial Monitor may be configured to send line ending characters when you send. Your sketch is looking for a NewLine ('\n') character to signal the end of input. Is your C# program sending a '\n' at the end of every line of output?

Thanks man, it works as intended now!
So basicly that means that the serial monitor puts a \n after every input?

Only if you tell it to.

That is one of the options. See the line-ending menu at the bottom of the Serial Monitor window:

  • No line ending
  • Newline
  • Carriage return
  • Both NL & CR

Thank you, it's all clear now!