How do you use #define?

static void Main(string[] args)
        {
#if DEBUG
            //this only compiles if in DEBUG
            Console.WriteLine("DEBUG")
#endif
#if !DEBUG
            //this only compiles if not in DEBUG
            Console.WriteLine("RELEASE")
#endif
            //This always compiles
            Console.ReadLine()
        }

#if definied(DEBUG)
  #define DEBUGLOG(message) printf("%s",message);
#else
  #define DEBUGLOG(message)
#end if

#if defined(DEBUG)
#undef DEBUG
#endif

#include <stdio.h>

#define PI 3.141592654

int main() {
    printf("%fn", PI);
}

#define PRINT(x) printf("%sn", (x))
/* Then you could say PRINT("Hello, world.n"); and it would
 * be replaced with printf("%sn", "Hello, Worldn");
 */

#ifdef linux
/* if linux is defined, this will be compiled. It will be left out if not. */
#endif

#if 0
/* This code will never be included for compilation. */
#endif

#define USEFOREACH

#if USEFOREACH
    foreach(var item in items)
     {
#else
    for(int i=0; i < items.Length; ++i)
     { var item = item[0];
#endif

       doSomethingWithItem(item);
     }

#define PLAYSOUNDS

#ifdef PLAYSOUNDS
// Do lots of funk to play a sound
return true;
#else
return true;