clash-of-styles-the-visitor

Visitor.cs

using System;

namespace Visitor
{
    class Program
    {
        public static void Main(string[] args)
        {
            var expression = new Addition(
                new MyInt(1), 
                new Addition(new MyInt(0), new MyInt(5)));
            
            var visitor = new CountZerosVisitor();
            var numberOfZeroes = expression.Accept(visitor);
            
            // Output: "The expression 1 + 0 + 5 has 1 zeroes."
            Console.WriteLine($"The expression {expression.Stringify()} has {numberOfZeroes} zeroes.");
        }
    }

    public interface IExpression
    {
        int Eval();
        string Stringify();
        T Accept<T>(IVisitor<T> visitor);
    }

    public class MyInt : IExpression
    {
        public int Val { get; } 
        
        public MyInt(int val)
        {
            Val = val;
        }

        public int Eval() => Val;
        public string Stringify() => Val.ToString();
        public T Accept<T>(IVisitor<T> visitor) => visitor.Visit(this);
    }
    
    public class Addition : IExpression
    {
        public IExpression Operand1 { get; }
        public IExpression Operand2 { get; }

        public Addition(IExpression operand1, IExpression operand2)
        {
            Operand1 = operand1;
            Operand2 = operand2;
        }

        public int Eval() => Operand1.Eval() + Operand2.Eval();
        public string Stringify() => $"{Operand1.Stringify()} + {Operand2.Stringify()}";
        public T Accept<T>(IVisitor<T> visitor) => visitor.Visit(this);
    }

    public interface IVisitor<out T>
    {
        T Visit(MyInt integer);
        T Visit(Addition addition);
    }

    public class CountZerosVisitor : IVisitor<int>
    {
        public int Visit(MyInt integer) => integer.Val == 0 ? 1 : 0;

        public int Visit(Addition addition) => 
            addition.Operand1.Accept(this) + addition.Operand2.Accept(this);
    }
}