Check out example codes for "hash sign c sharp". It will help you in understanding the concepts better.

Code Example 1

using System;
 public class Program
    {
        public static void Main(string[] args)
        {
        #if (!pi)
                    Console.WriteLine("i");
        #else 
                    Console.WriteLine("PI undefined");
        #endif
            Console.WriteLine("ok");
            Console.ReadLine();
        }
    }

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