// From the ECMA-334 C# Language specifications (section 17.8).
// Also in Hejlsberg et al., The C# Programming Language, 2ed.

using System; 

class CountPrimes
{
   static int Count(int max) {                 
      BitArray flags = new BitArray(max + 1);  
      int count = 1;                           
      for (int i = 2; i <= max; i++) {         
         if (!flags[i]) {
            for (int j = i * 2; j <= max; j += i) flags[j] = true;
            count++;                           
         }
      }
      return count;
   }
   static void Main(string[] args) {           
      int max = int.Parse(args[0]);            
      int count = Count(max);
      Console.WriteLine("Found {0} primes between 1 and {1}", count, max);
   }
}

 
 
 
 
 
 
 
Counts the number of primes less than max.
Makes a bit array with all
elements initialized to false.
Set flags[i*2], flags[i*3], ... to true.
 
 
Counts i as a prime.     
 
 
 
 
The main program which calls 
the Count method from above.