Editing Visitor pattern (section)

=== C# ===

This example declares a separate <code>ExpressionPrintingVisitor</code> class that takes care of the printing. If the introduction of a new concrete visitor is desired, a new class will be created to implement the Visitor interface, and new implementations for the Visit methods will be provided. The existing classes (Literal and Addition) will remain unchanged.
<syntaxhighlight lang="csharp">
using System;

namespace Wikipedia;

public interface Visitor
{
    void Visit(Literal literal);  
    void Visit(Addition addition);
}

public class ExpressionPrintingVisitor : Visitor
{
    public void Visit(Literal literal)
    {
        Console.WriteLine(literal.Value);
    }

    public void Visit(Addition addition)
    {
        double leftValue = addition.Left.GetValue();
        double rightValue = addition.Right.GetValue();
        var sum = addition.GetValue();
        Console.WriteLine($"{leftValue} + {rightValue} = {sum}");
    }
 }

public abstract class Expression
{
    public abstract void Accept(Visitor v);
   
    public abstract double GetValue();
}

public class Literal : Expression
{
    public Literal(double value)
    {
        this.Value = value;
    }

    public double Value { get; set; }

    public override void Accept(Visitor v)
    {
        v.Visit(this);
    }
 
    public override double GetValue()
    {
        return Value;
    }
}

public class Addition : Expression
{
    public Addition(Expression left, Expression right)
    {
        Left = left;
        Right = right;
    }

    public Expression Left { get; set; }
    public Expression Right { get; set; }

    public override void Accept(Visitor v)
    {
        Left.Accept(v);
        Right.Accept(v);
        v.Visit(this);
    }
  
    public override double GetValue()
    {
        return Left.GetValue() + Right.GetValue();
    }
}

public static class Program
{
    public static void Main(string[] args)
    {
        // Emulate 1 + 2 + 3
        var e = new Addition(
            new Addition(
                new Literal(1),
                new Literal(2)
            ),
            new Literal(3)
        );

        var printingVisitor = new ExpressionPrintingVisitor();
        e.Accept(printingVisitor);
        Console.ReadKey();
    }
}
</syntaxhighlight>