Visitor Design Pattern in C#

Key Components

1. Interfaces

• IVisitor: Represents the visitor interface with a Visit method that operates on IElement objects.
• IElement: Represents the element interface with an Accept method that accepts a visitor.

2. Concrete Visitor Classes

• Salesman: Implements the IVisitor interface. Defines behavior for visiting an IElement (specifically, a Kid object).
• Doctor: Another implementation of IVisitor, defining a different behavior for visiting a Kid.

3. Concrete Element Class

• Kid: Implements the IElement interface. Represents an object that can be visited by a visitor.

4. Object Structure

• School: Contains a collection of IElement objects (e.g., Kid instances) and allows visitors to visit each element.

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How the Code Works

1. Visitor Implementation (Salesman and Doctor)

• Both Salesman and Doctor implement the Visit method from the IVisitor interface.

• In the Visit method, they check if the IElement passed is a Kid object using the is keyword:

  if (element is Kid kid)
  {
      Console.WriteLine($"Salesman: {Name} gave a school bag to the child: {kid.KidName}");
  }

• Salesman: Prints a message about giving a school bag to the child.

• Doctor: Prints a message about performing a health checkup on the child.

2. Element Implementation (Kid)

• The Kid class implements the IElement interface.
• It contains a KidName property and an Accept method:

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

3. Object Structure (School)

• The School class contains a collection of IElement objects (e.g., Kid instances).
• It provides a method to allow a visitor to visit all elements in the collection:

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

4. Program Execution

• A School object is created, and Kid objects are added to it.
• Visitors (Salesman and Doctor) are instantiated and passed to the School's PerformOperation method:

  var school = new School();
  school.AddElement(new Kid("John"));
  school.AddElement(new Kid("Alice"));
  
  var salesman = new Salesman("Tom");
  var doctor = new Doctor("Dr. Smith");
  
  school.PerformOperation(salesman);
  school.PerformOperation(doctor);

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Flow of Execution

1. A School object is created, and Kid objects are added to it.
2. A Salesman and a Doctor are instantiated.
3. The PerformOperation method of School is called with each visitor.
4. Each visitor visits all the Kid objects in the school, performing their respective actions.

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Purpose of the Design

The Visitor Design Pattern allows adding new operations (e.g., Salesman and Doctor) without modifying the Kid class or other elements. This promotes the Open/Closed Principle, making the code more maintainable and extensible.

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