Finds all the prime numbers up to n using Sieve of Eratosthenes in C#. See http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes

SieveOfEratosthenes.cs

public static IList<int> SieveOfEratosthenes(int n) 
{
    if(n < 2)
    {
        throw new ArgumentOutOfRangeException();
    }

    var arr = Enumerable.Repeat(true, n+1).ToArray();

    for (int i = 2; i <= n; i++)
    {
        if(arr[i])
        {
            int j = 2;
            int mul = 0;
            while((mul = (j*i)) <= n)
            {
                arr[mul] = false;
                j++;
            }
        }
    }

    var primes = new List<int>();
    for (int i=2; i <= n; i++)
    {
        if(arr[i])
        {
            primes.Add(i);
        }
    }
    return primes;
}