example.cs

abstract class ShapeBase
{
    protected double Width = 0;
    protected double Height = 0;

    abstract public double GetArea();

    public Drawable Render(double area)
    {
        // ...
    }
}

class Rectangle : ShapeBase
{
    public void SetWidth(double width)
    {
        Width = width;
    }

    public void SetHeight(double height)
    {
        Height = height;
    }

    public double GetArea()
    {
        return Width * Height;
    }
}

class Square : ShapeBase
{
    private double Length = 0;

    public double SetLength(double length)
    {
        Length = length;
    }

    public double GetArea()
    {
        return Math.Pow(Length, 2);
    }
}

Drawable RenderLargeRectangles(Rectangle rectangles)
{
    foreach (rectangle in rectangles)
    {
        if (rectangle is Square)
        {
            rectangle.SetLength(5);
        }
        else if (rectangle is Rectangle)
        {
            rectangle.SetWidth(4);
            rectangle.SetHeight(5);
        }

        var area = rectangle.GetArea();
        rectangle.Render(area);
    }
}

var shapes = new[] { new Rectangle(), new Rectangle(), new Square() };
RenderLargeRectangles(shapes);