A simple C# class that provides prime numbers, in a List, and in a HashSet.
The reason I added the HashSet, is because it has O(1) time complexity for .Contains(), and it uses less space than a simple boolean sieve array.
The algorithm was taken from - http://stackoverflow.com/questions/2068372/fastest-way-to-list-all-primes-below-n-in-python/3035188#3035188 and is ported to C#.
Sample usage :
HashSet<int> set = Primes.GetPrimesHashSet(100000);
List<long> lst = Primes.GetPrimesList(1000000L);
Console.WriteLine("There are " + lst.Count + " primes < 100000");
Console.WriteLine("The sum of all primes < 100000 is " + lst.Sum());
Console.WriteLine("Is 99989 a prime ? " + set.Contains(99989));
Console.WriteLine("Is 99990 a prime ? " + set.Contains(99990));
Console.WriteLine("Is 99991 a prime ? " + set.Contains(99991));
Console.ReadKey();
/*
* Output :
------
There are 78498 primes < 100000
The sum of all primes < 100000 is 37550402023
Is 99989 a prime ? True
Is 99990 a prime ? False
Is 99991 a prime ? True
*/