Expectation

Any number which cannot be divisible by any other number except one and itself is called a prime number. In other words, any number which doesn't have more than 2 factors (1 and itself) is called a prime number. Examples: 2, 3, 11, 13, 19 and such For a given number n, we shall find out the total factors for the number and decide whether the number is a prime or not.

Logic

        public static bool DoIsPrimeNumber(int n)
        {
            // fetch the factors for n
            List<int> factors = DoGetFactors(n);

            return factors.Count > 2 ? false : true;
        }
        

How it works:

For a given number n, we'll find out all the factors and then see if the Count of the factors is more than 2 or not. If it is two we'll return it is a prime, since by definition a prime number can't have more than 2 factors.