Reply to https://giuliohome.wordpress.com/2016/01/05/algebraic-data-types

ShapeVisitor.cs

interface ShapeVisitor<A>
{
    // The implementation of ApplyRect defines handling for the
    // operation defined by this visitor in the case that the 
    // shape is a rectangle.
    public A ApplyRect(IRectangle rect);

    // The implementation of ApplyCircle defines handling for the
    // operation defined by this visitor in the case that the 
    // shape is a circle.
    public A ApplyCircle(ICircle circle);
}

public class AreaVisitor : ShapeVisitor<double>
{
    public double ApplyRect(IRectangle rect)
    {
        return rect.height * rect.width;
    }

    public double ApplyCircle(ICircle circle)
    {
        return Math.PI * circle.radius * circle.radius ;
    }
}

public class PerimeterVisitor: ShapeVisitor<double> 
{
    public double ApplyRect(IRectangle rect)
    {
        return 2 * (rect.height + rect.width);
    }

    public double ApplyCircle(ICircle circle)
    {
        return Math.PI * circle.radius * 2;
    }
}

abstract class IShape {
  public A accept<A>(ShapeVisitor<A> visitor);

  // This becomes essentially syntactic sugar; it need not be defined as a method on the interface.
  public double GetArea() 
  {
    return accept(new AreaVisitor());
  }
  
  // I will not define GetPerimeter here - there is no purpose. The user
  // can simply call myShape.accept(new PerimeterVisitor()) if they
  // wish that functionality. The point here is that the end user can "extend"
  // the interface by adding new Visitor implementations - they do not need
  // IShape to be altered.
}

class ICircle : IShape {
  public A accept<A>(ShapeVisitor<A> visitor) {
    return visitor.ApplyCircle(this);
  }
}

class IRectangle: IShape {
  public A accept<A>(ShapeVisitor<A> visitor) {
    return visitor.ApplyRect(this);
  }
}

Never mind! Found and commited the changes to switch to a visitor