Largest Prime Factor (PE3) using OOP
Project Euler questions are primarily about math.  This isn't really the place to practice object-oriented programming.  In fact, trying to model the problem literally will lead you to a suboptimal solution.

The "right" algorithm is very simple, and involves no primality testing.

I wouldn't bother with an IsFactor() method at all — if you just write it as a modulo test, there is no confusion about the order the parameters.

class ProjectEuler3
{
    private long number;

    public ProjectEuler3(long number)
    {
        this.number = number;
    }

    public long GetAnswer()
    {
        long n = this.number;
        while (n % 2 == 0)
        {
            n /= 2;
        }
        for (long factor = 3; factor &lt; n; factor += 2)
        {
            while (n % factor == 0)
            {
                n /= factor;
            }
        }
        return n;
    }

}