Polymorphism

static void Main()
    {
        try
        {
            List<Shape> shapes = new List<Shape>();
            shapes.Add(new Triangle(5, 3));
            shapes.Add(new Circle(4, 4));
            shapes.Add(new Rectangle(10, 4));
            //Polymorphism
            foreach (Shape item in shapes)
            {
                Console.WriteLine(string.Format("Surface : {0} -> {1}",item.GetType(),item.CalculateSurface()));
            }
        }
        catch (ArgumentException err)
        {
            Console.WriteLine(err.Message);
        }
    }

public abstract class Shape
{
    //fields
    private double width;
    private double height;
    //properties
    public double Width
    {
        get { return this.width; }
        private set
        {
            if (value <= 0)
            {
                throw new ArgumentException("Value must be positive!");
            }
            this.width = value;
        }
    }
    public double Height
    {
        get { return this.height; }
        private set
        {
            if (value <= 0)
            {
                throw new ArgumentException("Value must be positive!");
            }
            this.height = value;
        }
    }
    //abstract method
    public abstract double CalculateSurface();
    //constructor
    protected Shape(double width, double height)
    {
        this.Width = width;
        this.Height = height;
    }
}

public class Triangle : Shape
{
    public override double CalculateSurface()
    {
        return this.Height * this.Width / 2d;
    }

    public Triangle(double width, double height) : base(width,height)
    {

    }
}

public class Rectangle : Shape
{
    public override double CalculateSurface()
    {
        return this.Height * this.Width;
    }

    public Rectangle(double width, double height)
        : base(width, height)
    {

    }
}

public class Circle : Shape
{
    public override double CalculateSurface()
    {
        double radius = this.Width / 2d;
        return Math.PI * radius * radius;
    }

    public Circle(double width, double height)
        : base(width, height)
    {
        if (width != height)
        {
            throw new ArgumentException("In circle width must be equal to height!");
        }
    }
}