using System;

 public class Program

    {

        public static void Main(string[] args)

        {

            var theFoo = new Foo();

            int count = theFoo.Accept(new CountVisitor());

            string name = theFoo.Accept(new NameVisitor());

        }

    }

    public interface IVisitor<out TResult, in TVisitable> where TVisitable : IVisitable<TVisitable>

    {

        TResult Visit(TVisitable visitable);

    }

​

    public interface IVisitable<out TVisitable> where TVisitable : IVisitable<TVisitable>

    {

        TResult Accept<TResult>(IVisitor<TResult, TVisitable> visitor);

    }

​

    public class Foo : IVisitable<Foo>

    {

        public TResult Accept<TResult>(IVisitor<TResult, Foo> visitor) => visitor.Visit(this);

    }

​

    public class CountVisitor : IVisitor<int, Foo>

    {

        public int Visit(Foo visitable) => 42;

        public int Visit(Bar visitable) => 7;

    }

​

    public class NameVisitor : IVisitor<string, Foo>

    {

        public string Visit(Foo visitable) => "Chewie";

    }

​

    public class Bar : IVisitable<Bar>

    {

        public TResult Accept<TResult>(IVisitor<TResult, Bar> visitor) => visitor.Visit(this);

    }