public class Shape
{
    // A few example members
    public int X { get; private set; }
    public int Y { get; private set; }
    public int Height { get; set; }
    public int Width { get; set; }

    // Virtual method
    public virtual void Draw()
    {
        Console.WriteLine("Performing base class drawing tasks");
    }
}

public class Circle : Shape
{
    public override void Draw()
    {
        Console.WriteLine("Drawing a circle");
        base.Draw();
    }
}
public class Rectangle : Shape
{
    public override void Draw()
    {
        Console.WriteLine("Drawing a rectangle");
        base.Draw();
    }
}
public class Triangle : Shape
{
    public override void Draw()
    {
        Console.WriteLine("Drawing a triangle");
        base.Draw();
    }
}

var shapes = new List<Shape>
{
    new Rectangle(),
    new Triangle(),
    new Circle()
};

foreach (var shape in shapes)
{
    shape.Draw();
}
/* Output:
    Drawing a rectangle
    Performing base class drawing tasks
    Drawing a triangle
    Performing base class drawing tasks
    Drawing a circle
    Performing base class drawing tasks
*/