C# Visitor pattern compile-time safety example

functional_visitor_example.cs

// This visitor implementation attemps to make the best of both worlds, between C#'s functional features and compile-time safety.
// Using extension methods as visitors makes it a runtime error for there not to be an equivalent visitor for every subclass of the element hierarchy.
// This approach ensures compile-time safety, while minimizing boilerplate. It's a step closer to the ideal of "if it compiles, it works".

using System;

Room room = new Bathroom(10);

RoomVisitor visitorPrintSurface = new(
    bathroom => Console.WriteLine($"Surface of bathroom: {bathroom.Surface}"),
    bedroom => Console.WriteLine($"Surface of bedroom: {bedroom.Surface}"),
    kitchen => Console.WriteLine($"Surface of kitchen: {kitchen.Surface}"),
    hall => Console.WriteLine($"Surface of kitchen: {hall.Surface}"));
    
room.Accept(visitorPrintSurface);

sealed class RoomVisitor(
    Action<Bathroom> bathroom,
    Action<Bedroom> bedroom,
    Action<Kitchen> kitchen,
    Action<Hall> hall)
{
    public void Visit(Bathroom room) => bathroom(room);
    public void Visit(Bedroom room) => bedroom(room);
    public void Visit(Kitchen room) => kitchen(room);
    public void Visit(Hall room) => hall(room);
}

abstract record Room(double Surface)
{
     public abstract void Accept(RoomVisitor v);
}

sealed record Bathroom(double Surface) : Room(Surface)
{
    public override void Accept(RoomVisitor v) => v.Visit(this);
}

sealed record Bedroom(double Surface) : Room(Surface)
{
    public override void Accept(RoomVisitor v) => v.Visit(this);
}


sealed record Kitchen(double Surface) : Room(Surface)
{
    public override void Accept(RoomVisitor v) => v.Visit(this);
}

sealed record Hall(double Surface) : Room(Surface)
{
    public override void Accept(RoomVisitor v) => v.Visit(this);
}

visitor_example.cs

using System;

Room room = new Bedroom(10);

RoomVisitor visitorPrintSurface = new RoomVisitorPrintSurface();

room.Accept(visitorPrintSurface);

sealed class RoomVisitorPrintSurface : RoomVisitor
{
    public void Visit(Bathroom bathroom) => Console.WriteLine($"Surface of bathroom: {bathroom.Surface}");
    public void Visit(Bedroom bedroom) => Console.WriteLine($"Surface of bedroom: {bedroom.Surface}");
    public void Visit(Kitchen kitchen) => Console.WriteLine($"Surface of kitchen: {kitchen.Surface}");
}

interface RoomVisitor
{
    void Visit(Bathroom bathroom);
    void Visit(Bedroom bedroom);
    void Visit(Kitchen kitchen);
}

abstract record Room(double Surface)
{
     public abstract void Accept(RoomVisitor visitor);
}

sealed record Bathroom(double Surface) : Room(Surface)
{
    public override void Accept(RoomVisitor visitor) => visitor.Visit(this);
}

sealed record Bedroom(double Surface) : Room(Surface)
{
    public override void Accept(RoomVisitor visitor) => visitor.Visit(this);
}


sealed record Kitchen(double Surface) : Room(Surface)
{
    public override void Accept(RoomVisitor visitor) => visitor.Visit(this);
}

sealed record Hall(double Surface) : Room(Surface)
{
    public override void Accept(RoomVisitor visitor) => visitor.Visit(this);
}