{

    public enum ShapeType

    {

        Square,

        Circle

    }

    public interface IShape

    {

        void Draw();

        double Area { get; }

        ShapeType Type { get; }

    }

    public class Square : IShape

    {

        public double Length { get; }

        public Square(double length)

        {

            Length = length;

        }

        public void Draw() {}

        public double Area { get { return Length * Length; } }

        public ShapeType Type { get { return ShapeType.Square; } }

    }

    public class Circle : IShape

    {

        public double Radius { get; }

        public Circle(double radius)

        {

            Radius = radius;

        }

        public void Draw() {}

        public double Area { get { return Math.PI * Radius * Radius; } }

        public ShapeType Type { get { return ShapeType.Circle; } }

    }

    public class ShapeDataModel

    {

        private Dictionary<int, IShape> _shapes = new Dictionary<int, IShape>();

        private int _nextId = 1;

        public int AddShape(IShape shape)

        {

            int id = _nextId;

            _shapes[id] = shape;

            _nextId++;

            return id;

        }

        public IShape GetShape(int id)

        {

            return _shapes[id];

        }

    }

}

>>> sq1 = Example1.Square(2.)

>>> ci1 = Example1.Circle(1.3)

>>> sq2 = Example1.Square(2.5)

>>> m = Example1.ShapeDataModel()

>>> m.AddShape(sq1)

1

>>> m.AddShape(sq2)

2

>>> m.AddShape(ci1)

3

>>> m.GetShape(2)

<Example1.IShape object at 0x000002A3448A09A0>

>>> m.GetShape(2).Area

6.25

>>> m.GetShape(2).Length

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

AttributeError: 'IShape' object has no attribute 'Length'