Prime or Not...check and flag

Found this problem on one of the sites.
The idea is to identify a series of numbers as prime based on their ordinal and flag them accordingly based on the following conditions :

a) If prime, concatenate its ordinal in the array to the output string.
b) If not prime, concatenate zero.

Here is the algorithm in C#.

string statstring = String.Empty;
bool ipr = false;
double[] primetest = { 2, 65537, 2999999, 4999999, 98320414332827, 2 ^ 35 - 1, 34567875111, 12345678987654321 };

for (int i = 0; i < 8; i++)
{
ipr = IsPrimeNumber(primetest[i]);

if (ipr == true)
{
statstring += (i + 1).ToString();
}
else
{
statstring += "0";
}
}

MessageBox.Show(statstring);

static bool IsPrimeNumber(double num)
{

bool bPrime = true;
double factor = num / 2;
int i = 0;

for (i = 2; i <= Math.Sqrt(num); i++)
{
if ((num % i) == 0)
{
bPrime = false;
break;
}
}
return bPrime;
}

IsPrimeNumber is a function that determines if a given number is prime or not. Got this on the net, the only improvement being that I added break condition in the loop to exit if prime. (reduces the number of iterations).