Examples Of The Visitor Pattern In C# – A Practical Guide

This design pattern is out of this world! The Visitor Pattern is a commonly used design pattern in C# that helps traverse and process objects in an object structure. As a software developer, it’s helpful to understand design patterns in order to write scalable, flexible, and maintainable code. In this article, I’ll provide you with a practical and relatable guide with examples of the Visitor Pattern in C#.

I’ll highlight the benefits and drawbacks of using the Visitor Pattern, along with tips and best practices for its effective use. By the end of the article, you should have a deeper and practical understanding of the Visitor Pattern in C# which you can use to improve your programming skills.

Let’s dive in!

What is the Visitor Pattern?

The Visitor Pattern is a behavioral design pattern that allows for the separation of algorithms and object structure. The purpose of the pattern is to define a new operation on an object structure without changing the classes of said objects. This pattern is useful for complex operations that would be impractical to implement in the classes themselves.

Advantages of the Visitor Pattern

One of the advantages of the Visitor Pattern is its ability to define new operations on an existing object structure without modifying the objects themselves. This allows for greater flexibility in design and can simplify code maintenance. Another advantage is that the Visitor Pattern allows for the separation of concerns, with visitors handling distinct operations on each object within the structure.

If you think about these benefits from the perspective of extending functionality or providing alternate functionality in a system, we’re able to do so without modifying the existing implementations. We can simply add new behavior and not risk breaking existing implementations.

Drawbacks to the Visitor Pattern

However, the Visitor Pattern can also have downsides. When misused, it can add unnecessary complexity to code. Just because you have collections of objects that you want to operate on does not mean that you need to jump head-first into a full-fledged visitor pattern. It might just be overkill if you can iterate over the collection.

Additionally, adding new functionality with visitors can require updating code in multiple places, which can be a challenge in large codebases. I talk about this a lot with respect to enums and how they can be misused, but changes that require consuming code to update can be a heavy lift. We often talk about not wanting to “break open” a class to modify behavior, and while this looks opposite to that challenge, you’re breaking open many other classes to update them.

Scenarios and Examples of the Visitor Pattern in C#

Examples of when to use the Visitor Pattern include any situation in which a visitor needs to perform several different operations on a collection of objects while avoiding code duplication. But remember to balance the added complexity with your true needs! The Visitor Pattern can be especially useful in parsing and interpreting languages or DSLs.

When compared to other design patterns, the Visitor Pattern is most similar to the Decorator Pattern in that both patterns allow for adding functionality to an existing object without modifying its class. However, the Visitor Pattern is more focused on accumulating data and performing an operation on a group of objects.

The Visitor Pattern is particularly useful in scenarios where there are a large number of classes in an object hierarchy and new operations need to be added without modifying the classes. However, it can be problematic in scenarios where there are only a few classes in the object hierarchy and new operations are infrequently added. In these cases, the added complexity and number of classes required by the Visitor Pattern may outweigh its benefits.

How Does the Visitor Pattern Work in C#?

The Visitor Pattern is a behavioral design pattern that allows you to add new operations to an existing object structure, without modifying the classes that make up the structure. The Visitor Pattern relies on the use of two interfaces: Visitable and Visitor.

Step-by-Step Process of Using the Visitor Pattern in C#

The process for implementing the Visitor Pattern in C# involves the following steps:

• The Visitable interface defines an Accept method that takes a Visitor object as a parameter.
• The Visitor interface defines methods for each of the concrete Visitable classes.
• Concrete Visitable classes are created, each of which implements the Visitable interface.
• A concrete Visitor class is created, which implements the Visitor interface and defines what should happen when a method is called on a Visitable class.

While you don’t necessarily have to approach things in this exact order, these are the fundamental steps to putting together the Visitor Pattern in C#. Of course, a concrete code example makes this a bit more obvious!

Example code demonstrating the Visitor Pattern implementation in C#

    public interface class IVisitor<TElement>
        where TElement : IElement
    {
        void Visit(TElement element);
    }

    public sealed class ConcreteVisitor1 : IVisitor<IElement>
    {
        public void Visit(IElement element)
        {
            Console.WriteLine("{0} visited by {1}",
                element.GetType().Name, this.GetType().Name);
        }
    }

    public sealed class ConcreteVisitor2 : IVisitor<IElement>
    {
        public void Visit(IElement element)
        {
            Console.WriteLine("{0} visited by {1}",
                element.GetType().Name, this.GetType().Name);
        }
    }

    public interface IElement
    {
        void Accept(Visitor visitor);
    }

    public sealed class ConcreteElementA : Element
    {
        public override void Accept(Visitor visitor)
        {
            visitor.VisitConcreteElementA(this);
        }
    }

    public sealed class ConcreteElementB : IElement
    {
        public override void Accept(IVisitor visitor)
        {
            visitor.VisitConcreteElementB(this);
        }
    }

    public sealed class ElementCollection
    {
        private readonly List<IElement> _elements;

	public ElementCollection()
	{
            _elements = new List<IElement>();
	}

        public void Attach(IElement element)
        {
            elements.Add(element);
        }

        public void Detach(IElement element)
        {
            elements.Remove(element);
        }

        public void Accept(IVisitor visitor)
        {
            foreach (var element in elements)
            {
                element.Accept(visitor);
            }
        }
    }

The code above shows an ElementCollection which contains the elements that we will be visiting, along with a method called Accept. While this class is not necessarily required, it does keep consistent how elements are visited — in this case a simple foreach loop.

When Should I Use the Visitor Pattern in C#?

When deciding whether to use the Visitor Pattern in your C# software engineering project, there are several factors to consider. The Visitor Pattern is particularly useful when you have a complex object hierarchy and you want to implement operations on that hierarchy without changing the classes themselves.

For example, let’s say you have an abstract Document class and several concrete classes that inherit from the Document class, such as PDFDocument and WordDocument. You want to implement a feature that allows you to export each of these document types to a different file format, such as HTML or XML. Using the Visitor Pattern, you can define an ExportVisitor class that can visit each document and perform the necessary operations for the desired export format.

Coded Example of The Visitor Pattern in CSharp

Let’s check out another bit of example code demonstrating the visitor pattern in CSharp:

using System;
using System.Collections.Generic;

public interface IElement
{
    void Accept(IVisitor visitor);
}

public sealed class Animal : IElement
{
    public Animal(string name, string species, int healthStatus)
    {
        Name = name;
        Species = species;
        HealthStatus = healthStatus;
    }

    public string Name { get; set; }
    public string Species { get; set; }
    public int HealthStatus { get; set; }

    public void Accept(IVisitor visitor)
    {
        visitor.Visit(this);
    }
}

public interface IVisitor
{
    void Visit(Animal animal);
}

public sealed class Veterinarian : IVisitor
{
    public void Visit(Animal animal)
    {
        // Provide a health checkup and treatment
        if (animal.HealthStatus < 50)
        {
            Console.WriteLine($"{animal.Name} the {animal.Species} received medical attention.");
            animal.HealthStatus += 20;
        }
        else
        {
            Console.WriteLine($"{animal.Name} the {animal.Species} is healthy and doesn't need treatment.");
        }
    }
}

public sealed class Feeder : IVisitor
{
    public void Visit(Animal animal)
    {
        // Provide food and monitor feeding
        Console.WriteLine($"{animal.Name} the {animal.Species} has been fed.");
    }
}

public sealed class Zoo
{
    private List<Animal> _animals;

    public Zoo()
    {
        _animals = new List<Animal>();
    }

    public void AddAnimal(Animal animal)
    {
        _animals.Add(animal);
    }

    public void RemoveAnimal(Animal animal)
    {
        _animals.Remove(animal);
    }

    public void Accept(IVisitor visitor)
    {
        foreach (var animal in _animals)
        {
            animal.Accept(visitor);
        }
    }
}

public class Program
{
    public static void Main(string[] args)
    {
        Zoo zoo = new Zoo();
        zoo.AddAnimal(new Animal("Simba", "Lion", 60));
        zoo.AddAnimal(new Animal("Nemo", "Fish", 30));
        zoo.AddAnimal(new Animal("Bambi", "Deer", 75));

        IVisitor veterinarian = new Veterinarian();
        IVisitor feeder = new Feeder();

        Console.WriteLine("Veterinarian's Rounds:");
        zoo.Accept(veterinarian);

        Console.WriteLine("Feeding Time:");
        zoo.Accept(feeder);

        Console.ReadLine();
    }
}

In the above example, the Zoo class compared to the earlier generic example seems a bit more useful. I mentioned that you can do away with this class and still have the Visitor Pattern. However, having a class like this can help encapsulate and allow the reuse of your intended access pattern for visitors.

Other Considerations for When to Use the Visitor Pattern in C#

It’s worth noting that the Visitor Pattern is often used in conjunction with other design patterns, such as the Composite Pattern. The Visitor Pattern can make it easier to iterate over and perform operations on the nodes in a Composite structure.

Real-world scenarios in which the Visitor Pattern has been useful include complex software systems with large class hierarchies, graphics rendering and manipulation applications, and audio and video processing applications. It just might be the case that you’ve written some code that could have benefited from being a visitor pattern — Instead, you may have stacked extra functionality into the objects that you are visiting.

Overall, if you are dealing with a complex object hierarchy and need to perform multiple operations on the classes within that hierarchy, the Visitor Pattern can be a useful design pattern to consider. By separating the operations from the classes themselves, you can create code that is more modular, flexible, and extensible.

Wrapping up Examples of the Visitor Pattern in C#

The Visitor Pattern is an extremely useful design pattern for software developers to understand. The pattern helps to separate algorithms from the objects on which they operate, making it easier to add new functionality. By using the Visitor Pattern, software developers can implement complex functionality without modifying the code for the objects themselves.

By walking through some examples of the Visitor Pattern in CSharp, I’m hopeful that you could see the capabilities it adds. I also hope that you notice how the extra code could contribute to a bit of over-engineering if you’re using the Visitor Pattern in C# when you really only need a simple solution in your loop.

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Nick Cosentino Principal Software Engineering Manager

Principal Software Engineering Manager at Microsoft. Views are my own.

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