IsPrime.cs

var sw = System.Diagnostics.Stopwatch.StartNew();
int total = 0;
for (int i = 1; i <= 20_000_000; i++) {
    if (IsPrime(i)) {
        total++;
    }
}
sw.Stop();

Console.WriteLine($"{total} in {sw.Elapsed} sec");


bool IsPrime(int number) {
    if ((number % 2) == 0) {
        return number == 2;
    }

    var sqrt = (int)Math.Sqrt(number);
    for (int t = 3; t <= sqrt; t += 2) {

        if ((number % t) == 0) {
            return false;
        }
    }
    return number > 1;
}