Use #define and #undef to control the program logic : Undef : Preprocessing Directives C# Examples

The #undef directive removes a previously defined definition.
The #undef directive "undefines" a symbol.

The general form for #undef is




    
#undef  symbol
    
   
  
   





    
#define  win2000
#define  release
#undef    win98

using  System;
using  System.Diagnostics;


class  MainClass
{
        public  static  void  Main()
        {
                string  platformName;

                #if  winXP              //  Compiling  for  Windows  XP
                        platformName  =  "Microsoft  Windows  XP";
                #elif  win2000      //  Compiling  for  Windows  2000
                        platformName  =  "Microsoft  Windows  2000";
                #elif  winNT          //  Compiling  for  Windows  NT
                        platformName  =  "Microsoft  Windows  NT";
                #elif  win98          //  Compiling  for  Windows  98
                        platformName  =  "Microsoft  Windows  98";
                #else                      //  Unknown  platform  specified
                        platformName  =  "Unknown";
                #endif

                Console.WriteLine(platformName);

        }
}