for (int i = 0; i < poly.Length; i++)
    {
        Point newPoint = new Point(poly[i].X, poly[i].Y);

        if (newPoint.X > oldPoint.X)
        {
            p1 = oldPoint;
            p2 = newPoint;
        }
        else
        {
            p1 = newPoint;
            p2 = oldPoint;
        }

        if ((newPoint.X < p.X) == (p.X <= oldPoint.X)
            && ((long)p.Y - (long)p1.Y) * (long)(p2.X - p1.X) < ((long)p2.Y - (long)p1.Y) * (long)(p.X - p1.X))
        {
            inside = !inside;
        }

        oldPoint = newPoint;
    }