C# beginner

OK, first semester in C# and just starting to work on it.  Haven't programmed in a long time, kinda stumped on this one.  The assignment is to determine if a number is prime, and if its not, to factor it.  I know what a prime number is in my head, but I don't really know how to put that into code.

Thanks.

(defun primep (number)
  (when (> number 1)
    (loop for fac from 2 to (isqrt number) never (zerop (mod number fac)))))

(defun next-prime (number)
  (loop for n from number when (primep n) return n))

(defmacro do-primes (var-and-range &rest body)
  (let ((var (first var-and-range))
        (start (second var-and-range))
        (end (third var-and-range)))
    `(do ((,var (next-prime ,start) (next-prime (1+ ,var))))
         ((> ,var ,end))
       ,@body)))