var number;
Console.WriteLine("Enter a number:");
number = Convert.ToInt32(Console.ReadLine());
if (IsPrime(number))
{
    Console.WriteLine("**" + number + " is a prime number**");
}
else
{
    Console.WriteLine("**" + number + " is not a prime number**");
}

public static bool IsPrime(int number)
{
    if (number <= 1) return false;
    if (number == 2) return true;
    if (number % 2 == 0) return false;
    var boundary = (int)Math.Floor(Math.Sqrt(number));
    for (int i = 3; i <= boundary; i += 2)
        if (number % i == 0)
            return false;
    return true;
}