gistfile1.cs

public abstract class Hole
{
    public static readonly float LineWidth = 2f;

    protected Hole(int radius)
    {
        Radius = radius;
    }

    public PointF Point { get; set; }
    public UIBezierPath Path { get; protected set; }

    protected int Radius { get; private set; }

    public virtual void Draw(RectangleF parentFrame)
    {
        var rect = CalculateRectangleForHole(parentFrame);
        Path = UIBezierPath.FromOval(rect);

        SetStrokeColour();
        Path.LineWidth = LineWidth;
        Path.Stroke();
    }

    protected abstract void SetStrokeColour();

    protected virtual RectangleF CalculateRectangleForHole(RectangleF parentFrame)
    {
        var offset = Radius;
        var y = parentFrame.Height - Point.Y - offset;
        var x = Point.X - offset;

        var length = Radius * 2f;
        return new RectangleF(x, y, length, length);
    }
}