public abstract class FilterBase<T>

{

    public abstract IQueryable<T> GetFilter(IQueryable<T> query);

}

abstract class Shape

{

    public abstract double GetArea();

}

​

class Triangle : Shape

{

    double baseLength;

    double height;

    public Triangle(double baseLength, double height)

    {

        this.baseLength = baseLength;

        this.height = height;

    }

    public override double GetArea()

    {

        return 0.5 * baseLength * height;

    }

}

​

class Rectangle : Shape

{

    double width;

    double height;

    public Rectangle(double width, double height)

    {

        this.width = width;

        this.height = height;

    }

    public override double GetArea()

    {

        return width * height;

    }

}

​

class Program

{

    static void Main(string[] args)

    {

        Shape triangle = new Triangle(4, 5);

        Shape rectangle = new Rectangle(3, 6);

        Console.WriteLine("Triangle area: " + triangle.GetArea());

        Console.WriteLine("Rectangle area: " + rectangle.GetArea());

    }

}