素数有0个自变量的问题

我得到一个错误"当我在事件处理程序中调用IsPrime时，方法'IsPrime'的重载不需要0个参数？

public bool IsPrime(int testNum)
{
    // return True if argument is prime number
    // return false otherwise
    // A prime number is a natural number that has exactly two divisors, 1 and the number its self.
    // 
    if (testNum == 1) return false; // by definition of Prime numbers, 1 is not a prime
    if (testNum == 2) return true; // short circuit out, we know that 2 is the first prime number
    for (int i = 2; i < testNum; ++i)  {
       if (testNum % i == 0)  return false;
    }
    return true;
}