LocalFunctions.cs

IEnumerable<int> GiveMePrimeNumbers2(IEnumerable<int> numbers)
{
    return numbers.Where(s => isPrime(s));

    bool isPrime(int number)
    {
        if (number < 2)
        {
            return false;
        }

        var boundary = (int) Math.Floor(Math.Sqrt(number));
        for (int i = 2; i <= boundary; ++i)
        {
            if (number % i == 0)
            {
                return false;
            }
        }
        return true;
    }
}