How to Write a function that counts the number of primes in the range 1-N?

Please check following link for primality test. http://en.wikipedia.org/wiki/Primality_test

Following code is not in the optimized form...and it may contains some error please check it carefully before using ... int[] numbers = { 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24 }; System.Collections.ArrayList PrimeNumbers = new System.Collections.ArrayList(); Boolean isPrime = false; for (int loop = 0; loop < numbers.Length; loop++) { int intNumberToTest = numbers[loop]; for (int intTest = 2; intTest <= Convert.ToInt32(Math.Sqrt(intNumberToTest)); intTest++) { if (intNumberToTest % intTest == 0) { isPrime = false; break; } else isPrime = true; } if(isPrime) PrimeNumbers.Add(intNumberToTest); }