I am using this header: http://playground.arduino.cc/Interfacing/CPPWindows

My main:

#include <iostream>
#include <stdio.h>
#include <tchar.h>
#include "SerialClass.h"
#include <string>

int main(int argc, _TCHAR* argv[])
{
    Serial* SP = new Serial("\\\\.\\COM4");

    if (SP->IsConnected())
        printf("We're connected\n");

    std::string tryStr;
    int value;
    DWORD bytesSend;



    /* Loop */
    while(SP->IsConnected())
    {
         /* Take a value from user */
         printf("Value: "); scanf("%d",&value); 

         /* Send Arduino */
         BOOL retVal = WriteFile(SP->hSerial, &value, sizeof(value), &bytesSend, NULL);

        /* I have no idea what is this for? (SP->status.cbInQue) */
        ClearCommError(SP->hSerial, &SP->errors, &SP->status);

        /* Read string from arduino */
        DWORD bytesRead;
        tryStr.assign("");
        bool check = true;
        char tempCharacter;
        while (check==true){

            ReadFile(SP->hSerial, &tempCharacter, sizeof(char), &bytesRead, NULL);
            tryStr += tempCharacter;

            if(tempCharacter == '$'){
                check = false;
            }
        }

        /* Print the string */
        std::cout << tryStr  << std::endl << std::endl ;
    }
    return 0;
}

My Arduino Code:

int LedPin= 10; 
int value;
String  incomingData = "hello$";

void setup (){
  Serial.begin(9600); 
  pinMode(LedPin, OUTPUT); 
}

void loop (){

  while(!(Serial.available() > 0)){
    /*Wait until take data */
  }
  
  value=Serial.read(); 

  if (value == 20)
  {
      digitalWrite (LedPin, HIGH); 
      incomingData.setCharAt(0,'o');
      incomingData.setCharAt(1,'p');
      incomingData.setCharAt(2,'e');
      incomingData.setCharAt(3,'n');
      incomingData.setCharAt(4,'n');
  
      Serial.print(incomingData);
  }
  else if (value == 15)
  {
      digitalWrite (LedPin, LOW);
      incomingData.setCharAt(0,'c');
      incomingData.setCharAt(1,'l');
      incomingData.setCharAt(2,'o');
      incomingData.setCharAt(3,'s');
      incomingData.setCharAt(4,'e');
  
      Serial.print(incomingData);
  }
  

}

This code works perfectly. Output:

We're connected Value: 20 openn$

Value: 15 close$

Value: 20 openn$

Value: 20 openn$

Value: 15 close$

Value: 15 close$

Value: 20 openn$

But now I want to add another condition in my Arduino code:

int LedPin= 10; 
int value;
String  incomingData = "hello$";

void setup (){
  Serial.begin(9600); 
  pinMode(LedPin, OUTPUT); 
}

void loop (){

  while(!(Serial.available() > 0)){
    /*Wait until take data */
  }

  value=Serial.read(); 

  if (value == 20)
  {
      digitalWrite (LedPin, HIGH); 
      incomingData.setCharAt(0,'o');
      incomingData.setCharAt(1,'p');
      incomingData.setCharAt(2,'e');
      incomingData.setCharAt(3,'n');
      incomingData.setCharAt(4,'n');

      Serial.print(incomingData);
  }
  else if (value == 15)
  {
      digitalWrite (LedPin, LOW);
      incomingData.setCharAt(0,'c');
      incomingData.setCharAt(1,'l');
      incomingData.setCharAt(2,'o');
      incomingData.setCharAt(3,'s');
      incomingData.setCharAt(4,'e');

      Serial.print(incomingData);
  }
  else{                                      // new line added
                                             // new line added
      incomingData.setCharAt(0,'g');         // new line added
      incomingData.setCharAt(1,'g');         // new line added
      incomingData.setCharAt(2,'g');         // new line added
      incomingData.setCharAt(3,'g');         // new line added
      incomingData.setCharAt(4,'g');         // new line added
                                             // new line added
      Serial.print(incomingData);            // new line added
  }                                          // new line added

}

But it didn't work. ScreenShot:

What went wrong? The led blinked correctly but the output string is wrong.